Controlling the isomerization rate of Azo-BF2 switches using aggregation

ABSTRACT

Provided herein are photochromic organic compounds of Formula I or Formula II, which are useful as molecular switches capable of being triggered via a cis/trans isomerization process. Methods of using the molecular switch compounds to form photopharmaceutical compounds that may be used to provide selective spatiotemporal activation of pharmaceutical agents are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 15/056,580, with a 371(c) date of Feb. 29, 2016, which is a U.S. National Stage Application under 35 U.S.C. § 371 of International Application No. PCT/US2014/052983, filed Aug. 27, 2014, which claims the benefit of priority from U.S. Provisional Patent Application No. 61/870,572, filed Aug. 27, 2013. Each of these applications is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant Number W911NF-15-1-0587 awarded by the Department of Defense Army Research Office. The Government has certain rights in the invention.

BACKGROUND

Photoreversible, organic compounds are especially attractive materials for use as molecular switches. Photoreversible compounds are stimulated by light, i.e., they provide reversible switching processes based on photochemically induced interconversions. Photochromism, a reversible change induced by light irradiation between two states of a molecule having different electromagnetic absorption spectra, is commonly associated with such photoreversible systems. Photochromic switching processes are typically based on photocyclization of isomers, the conversion of olefinic (cis/trans) isomers, photoinduced electron transfer, and keto-enol tautomerism.

Photochromic molecular switches based on the trans→cis isomerization of azobenzene are useful for many purposes, including industrial dyes, actuators, nonlinear optical devices, liquid crystals, molecular machines, ion channel modulators, and the like. Ultraviolet (UV) light often is used to induce the trans→cis isomerization. However, UV light can be harmful in certain applications, especially those involving in vivo systems. Hence, there is a significant need in the art for photochromic molecular switches that can be toggled between cis and trans states using lower energy electromagnetic irradiation with less scattering and that can penetrate tissues more easily (i.e., red and NIR light).

SUMMARY OF THE INVENTION

Provided herein are compounds, photochromic materials comprising such compounds, and methods of using such compounds as molecular switches. In an embodiment, the compounds may be used in photopharmaceutical compounds as molecular switches that allow for selective spatiotemporal activation of pharmaceutical agents.

Accordingly, in one aspect, provided herein is a compound of Formula II:

or a salt thereof, wherein

N═N—R² can be oriented cis or trans to the tricycle;

R¹ is H, CN, CO₂H, CO₂(C₁₋₆-alkyl), C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl);

R² is C₆₋₁₉-aryl or C₃₋₁₄-heteroaryl, wherein the C₆₋₁₉-aryl or C₃₋₁₄-heteroaryl is independently substituted one or more times at the para and/or ortho position with C₁₋₆-alkyl, C₆₋₁₉-aryl, C₃₋₁₄-heteroalkyl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl); and

R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, H, C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, NHC(O)(C₁₋₆-alkyl) or a group corresponding to a small molecule pharmaceutical; or

R³ and R⁴, R⁴ and R⁵, R⁵ and R⁶ or R⁷ and R⁸ can, when taken together, form a fused aryl, fused heteroaryl, fused C₃₋₆-cycloalkyl, or fused heterocycle, wherein the fused aryl, fused heteroaryl, fused cycloalkyl, or fused heterocycle can be optionally substituted one or more times with C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl).

In another aspect, provided herein is a compound of the Formula I:

or a salt thereof, wherein

N═N—R² can be oriented cis or trans to the tricycle;

R¹ is H, CN, CO₂H, CO₂(C₁₋₆-alkyl), C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl);

R² is unsubstituted C₆₋₁₉-aryl or unsubstituted C₃₋₁₄-heteroaryl; and

R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, H, C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, NHC(O)(C₁₋₆-alkyl) or a group corresponding to a small molecule pharmaceutical; or

R³ and R⁴, R⁴ and R⁵, R⁵ and R⁶ or R⁷ and R⁸ can, when taken together, form a fused aryl, fused heteroaryl, fused C₃₋₆-cycloalkyl, or fused heterocycle, wherein the fused aryl, fused heteroaryl, fused cycloalkyl, or fused heterocycle can be optionally substituted one or more times with C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁-6-alkyl)₂, or NHC(O)(C₁₋₆-alkyl).

In certain embodiments of Formula I and Formula II, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, H.

In certain other embodiments of Formula I and Formula II, R¹ is CN.

In certain embodiments of Formula I, R² is unsubstituted phenyl.

In certain embodiments of Formula II, R² is phenyl, wherein the phenyl is substituted at the para position. In certain other embodiments of Formula II, R² is phenyl, wherein the phenyl is substituted at the para position with N(CH₃)₂, OCH₃, piperazinyl, (N-methyl)piperazinyl, piperidinyl, morpholinyl or a group corresponding to a cholesterol molecule.

In some embodiments of Formula I and Formula II, N═N—R² is oriented cis to the tricycle. In other embodiments of Formula I and Formula II, N═N—R² is oriented trans to the tricycle.

In certain embodiments of Formula I and Formula II, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, a bond to a pharmaceutical agent.

In one aspect, provided herein is a photochromic molecular switch comprising at least one compound of Formula I or a salt thereof. In another aspect, provided herein is a photochromic molecular switch comprising at least one compound of Formula II or a salt thereof.

In yet another aspect, provided herein is a method of switching a molecular switch, wherein the molecular switch comprises a compound of Formula I or a salt thereof; wherein the method comprises applying electromagnetic radiation to the molecular switch at a first wavelength effective to cause the trans→cis isomerization of the compound of Formula I or salt thereof; or applying electromagnetic radiation to the molecular switch at a second wavelength effective to cause the cis→trans isomerization of the compound of Formula I or salt thereof; or a combination thereof.

In still another aspect, provided herein is a method of switching a molecular switch, wherein the molecular switch comprises a compound of Formula II or a salt thereof; wherein the method comprises applying electromagnetic radiation to the molecular switch at a first wavelength effective to cause the trans→cis isomerization of the compound of Formula II or salt thereof; or applying electromagnetic radiation to the molecular switch at a second wavelength effective to cause the cis→trans isomerization of the compound of Formula II or salt thereof; or a combination thereof.

In certain embodiments, the molecular switch comprising the salt of Formula I or the salt of Formula II is reacted with a pharmaceutically acceptable salt to form a photopharmaceutical compound. In certain embodiments, the pharmaceutically acceptable salt is a salt of a small molecule pharmaceutical.

In one embodiment, the molecular switch comprising the compound of Formula I or the compound of Formula II or a photopharmaceutical compound thereof is stable in water or biological fluid. In another embodiment, the molecular switch comprising the compound of Formula I or the compound of Formula II or a photopharmaceutical compound thereof does not substantially hydrolyze in water or biological fluid.

In certain embodiments, a method of treating a patient in need of therapy comprises administering a therapeutically effective amount of a photopharmaceutical compound to a patient in need thereof; and applying electromagnetic radiation to a tissue comprising the photopharmaceutical compound at a first wavelength effective to cause the trans→cis isomerization or applying electromagnetic radiation to the photopharmaceutical compound at a second wavelength effective to cause the cis→trans isomerization or a combination thereof.

In one embodiment of the methods of switching the molecular switch, the electromagnetic radiation is generated by an infrared light source and/or a visible light source.

In another embodiment of the methods of switching the molecular switch, the electromagnetic radiation is generated by a visible light source.

In one embodiment, a method of controlling the isomerization rate of a compound is disclosed, wherein the method include (a) selecting a compound capable of isomerizing between a thermally stable isomer and a thermodynamically less stable (kinetic) isomer, (b) providing a solution of the compound having a preset concentration of the compound, (c) exposing the solution to electromagnetic radiation having a wavelength effective to cause isomerization of the compounds in the solution to the kinetic isomer, and (d) shielding the solution from electromagnetic radiation to allow for thermal relaxation of the kinetic isomer, while changing the concentration of the solution (either dilution or further concentration), thereby controlling the isomerization rate of the compound. In one aspect, the compound is of Formula II. In another aspect, step (c) takes place in a mammalian cell. In another aspect, step (c) takes place in the body of a human. In another aspect, the electromagnetic radiation in step (c) causes isomerization of at least 10%, 50%, 60%, 70%, 80%, or at least 90% of the compounds in the solution to the kinetic isomer.

In one embodiment, the thermal relaxation is concentration-dependent. In another embodiment, the concentration of the compound is between 1 mM and 5 M, between 3 mM and 1 M, or between 5 mM and 1 M.

In another embodiment, the preset concentration may be determined by (e) measuring the number of molecules of the compound per unit volume (concentration) that provides a desired half-life of the kinetic isomer, wherein the step (e) is performed before step (b).

In another embodiment, the solution of the compound may be in a liquid or a solid state, and the solvent may be a polar solvent, a non-polar solvent, a gel or a solid matrix. By way of examples, suitable solvents may include but are not limited to 1,2dichloroethane, toluene, benzene, Tetrahydrofuran (THF), diethylether, ethylacetate, acetonitrile, 1,4dioxane,n-methylpyrrolidone, or Dimethylformamide (DMF).

In one embodiment, the concentration of the kinetic isomer is increased at least 10-fold after the solution is exposed to the wavelength effective to cause isomerization. In another embodiment, the half-life of the kinetic isomer is increased by a factor of at least 1000 at the preset concentration. In one aspect, the step of selecting a concentration of the compound may include a step of extrapolating from a graph of concentration versus half-life or isomerization rate. In another aspect, the step of selecting a concentration of the compound may include utilizing a look-up table of concentration versus half-life or isomerization rate. In another aspect, the step of exposing the solution to electromagnetic radiation having the wavelength effective to cause isomerization may include providing a filter between a source of the electromagnetic radiation and the solution.

In one embodiment, the electromagnetic radiation may be generated by an infrared light source and/or a visible light source. In one aspect, the wavelength (λ) may be between 400 nm and 1000 nm, between 450 nm and 900 nm, or between 500 nm and 700 nm.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a schematically illustrates a photopharmaceutical compound comprising a pharmaceutical agent (circle, star, triangle, square) bound to Compound 1, where activation of the pharmaceutical agent is triggered by isomerization of Compound 1.

FIG. 1b shows the UV/vis spectral changes upon the photo-isomerization of Compound 1 in deoxygenated CH₂Cl₂(0.1 mM). The black trace is of Compound 1 equilibrated in the dark (mainly trans), which upon irradiation at λ₁=570 nm gives the cis photo stationary state (PSS, blue trace), which when irradiated at λ₂=450 nm gives the trans PSS (red trace).

FIG. 1c shows multiple isomerization cycles of Compound 1 (0.1 mM) in CH₂Cl₂ (not deoxygenated) after alternative irradiation at λ₁=570 nm (red trace) and λ₂=450 nm (black trace).

FIG. 2 shows the visible light-induced trans/cis isomerization of Compound 1.

FIG. 3 shows the calculated (B3LYP/6-311++G**) molecular orbital energy levels, transition energies and oscillator strengths of the n→π* and π_(nb)→π* transitions of the trans isomer of Compound 1.

FIG. 4 shows the optimized structure of Compound 1-trans.

FIG. 5 shows the optimized structure of Compound 1-cis.

FIG. 6 shows the HOMO of Compound 1-trans (0.05 isosurface).

FIG. 7 shows the HOMO-2 of Compound 1-trans (0.05 isosurface).

FIG. 8 shows the calculated UV/Vis spectrum of Compound 1-trans.

FIG. 9 shows the calculated UV/Vis spectrum of Compound 1-cis.

FIG. 10 shows the calculated UV/Vis spectrum of Na-Azo complex (trans).

FIG. 11 shows the calculated UV/Vis spectrum of Na-Azo complex (cis).

FIG. 12 shows the calculated substituent effects (X) on HOMO-LUMO energies of BF₂-Azo compounds. Substitution of H by the 7-acceptor substituent CN decreases the HOMO-LUMO energy gap by stabilizing the HOMO more than the LUMO. Substitution of H by a π-donor OH substituent inductively stabilizes both HOMO and LUMO, but the π-donor component strongly destabilizes the HOMO, resulting in a lower overall HOMO-LUMO energy gap.

FIG. 13 shows the calculated substituent effects (X) on UV/Vis spectra of BF₂-Azo complexes (trans).

FIG. 14 depicts the idealized MO picture for the π-orbitals of the N—C—C—N—N skeleton of the azo compounds. Energies increase with the number of nodes. The HOMO→LUMO transition is from a MO that is π-nonbonding to one which is π*-antibonding between either the NN or CN subunits, as shown.

FIG. 15 demonstrates the H-Azo trans vs. cis MO Levels: stripped molecules. The π-MOs follow the same nodal pattern as those in FIG. 14. Lone pair MOs rise in energy from trans→cis due to greater electron-electron repulsion in cis configuration.

FIG. 16 demonstrates the H-Azo vs. BF₂-Azo MO Levels: stripped molecules. BF₂ coordination lowers all energy levels by electron withdrawal; lone pair levels are lowered even more significantly by coordination and become bonding instead of non-bonding. The HOMO→LUMO transition is from an MO that is π-nonbonding to one which is π*-antibonding between either the NN or CN subunits. Unlike the H-compound, only the NN linkage is subject to trans→cis isomerization (switching) in the BF₂ compound; the rest of the system is clamped in position by coordination to the BF₂ group.

FIG. 17 shows the calculated BF₂-Azo trans vs. cis MO Levels: stripped molecules. Higher energy HOMO→LUMO transitions in the cis isomer result from the stabilization of the HOMO in the latter, without significant stabilization of the corresponding LUMO.

FIG. 18 depicts the BF₂-Azo trans vs. cis HOMO→LUMO Levels: full molecules. Higher energy HOMO→LUMO transitions in the cis isomer result from the stabilization of the HOMO in the latter, without significant stabilization of the corresponding LUMO.

FIG. 19 (a) The UV/Vis spectral changes upon the photoisomerization of 2 in CH₂Cl₂ (0.2 mM). The black trace is of 2 equilibrated under dark (mainly trans), which upon irradiation at λ=630 nm gives the cis PSS (blue trace). Irradiating the latter at λ=490 nm gives the trans PSS (red trace). (b) The UV/Vis spectral changes upon the photoisomerization of 3 in CH₂Cl₂ (0.1 mM). The black trace is of 3 equilibrated under dark, which upon irradiation at λ=640 nm gives the cis PSS (blue trace). Irradiating the latter at λ=490 nm gives the trans PSS (red trace).

FIG. 20. (a) NIR light-induced trans/cis isomerization of 4. (b) The UV/Vis spectral changes upon the photoisomerization of 4 in CH₂Cl₂ (0.2 mM). The black trace is of 4 equilibrated under dark (mainly trans), which upon irradiation at A=710 nm gives the cis populated state (red trace).

FIG. 21. UV/Vis spectral changes upon photoisomerization of (a) 5, (b) 6 and (c) 7. The black trace is equilibrated complex in the dark (mainly trans), which upon irradiation at λ=710 nm gives the cis populated state (red trace); (d) Overlay of the π-π* absorption bands of the trans isomers of 5-7, represented by black, blue and red lines, respectively.

FIG. 22. (a) Switching cycles of 4 in acetonitrile:PBS buffer (1:1) monitored by following the absorbance at λ=681 nm (black trace) upon irradiation at λ=710 nm then leaving in the dark. (b) UV/Vis spectra following an acetonitrile:PBS buffer (1:1) solution of the azo-BF₂ complex 4 at 25° C. The interval between each scan is 20 min.

FIG. 23. ORTEP drawing (50% probability ellipsoids) of the crystal structures of (a) 2 and (b) 4. The hydrogen atoms were removed for clarity.

FIG. 24. (a) ¹H NMR and (b) ¹³C NMR spectra of Hydrazone 2 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

FIG. 25. (a) ¹H NMR (b) ¹³C NMR and (c) ¹⁹F NMR spectra of 2-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K.

FIG. 26. ¹H NMR spectrum of 2 after being stored in the dark. The equilibrated mixture of 2 was determined to have an isomer ratio of 93:7 (trans:cis).

FIG. 27. ¹H NMR spectra of the PSS of 2 at (a) 490 nm and (b) 630 nm in CD₂Cl₂ at 294 K. The PSS isomer ratios of 92±1% trans at λ_(irr)=490 nm, and 96±1% cis at λ_(irr)=630 nm are the averages of three experiments.

FIG. 28. a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 3 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

FIG. 29. a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 3-cis (contains a small percentage of the trans isomer) in CD₂Cl₂ at 294 K.

FIG. 30. ¹H NMR spectrum of 3 after being stored in the dark. The equilibrated mixture of 3 was determined to have an isomer ratio of 58:42 (trans:cis).

FIG. 31. ¹H NMR spectra of the PSS of 3 at a) 490 nm and b) 640 nm in CD₂Cl₂ at 294 K. The PSS isomer ratios of 56±1% trans at λ_(irr)=490 nm, and 79±1% cis at λ_(irr)=640 nm are the averages of three experiments.

FIG. 32. a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 4 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

FIG. 33. a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 4-trans in CD₂Cl₂ at 294 K.

FIG. 34. ¹H NMR spectrum of 4 after being stored in the dark. The equilibrated mixture of 4 was determined to have an isomer ratio of 97:3 (trans:cis).

FIG. 35. ¹H NMR spectrum recording the lowest estimation of PSS of 4 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 63±1% cis at λ_(irr)=710 nm is the average of three experiments.

FIG. 36. a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 5 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

FIG. 37. a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 5-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K.

FIG. 38. ¹H NMR spectrum of 5 after being stored in the dark. The equilibrated mixture of 5 was determined to have an isomer ratio of 98:2 (trans:cis).

FIG. 39. ¹H NMR spectrum recording the lowest estimation of PSS of 5 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 28±1% cis at λ_(irr)=710 nm is the average of three experiments.

FIG. 40. a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 7 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

FIG. 41. a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 7-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K.

FIG. 42. ¹H NMR spectrum of 7 after being stored in the dark. The equilibrated mixture of 7 was determined to have an isomer ratio of 94:6 (trans:cis).

FIG. 43. ¹H NMR spectrum recording the lowest estimation of PSS of 7 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 82±1% cis at λ_(irr)=710 nm is the average of three experiments.

FIG. 44. a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 6 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

FIG. 45. a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 6-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K.

FIG. 46. ¹H NMR spectrum of 6 after being stored in the dark. The equilibrated mixture of 7 was determined to have an isomer ratio of 97:3 (trans:cis).

FIG. 47. ¹H NMR spectrum recording the lowest estimation of PSS of 6 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 49±1% cis at λ_(irr)=710 nm is the average of three experiments.

FIG. 48. Plots of Absorbance versus time showing the first-order decays of (a) 2 and (b) 3 in degassed CH₂Cl₂ at 294 K starting from PSS630 and PSS640, respectively. The black lines represent the experimental results while the red lines represent the theoretical first-order fits.

FIG. 49. Plots of cis isomer ratio (%) versus time showing the first-order decay of 4 in CD₂Cl₂ at 294 K starting from its PSS710. The black dots represent the experimental results while the red line represents the theoretical first-order fit.

FIG. 50. ¹H NMR spectra of a) 4 in CD₃CN, b) 4 with water in CD₃CN and c) Hydrazone 4 in CD₃CN at 294 K.

FIG. 51. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 4 incubated with 10 mM GSH at 25° C. The interval between each scan is ˜20 min. A half-life of 2.5 h is observed.

FIG. 52. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 5 at 25° C. The interval between each scan is ˜20 min. A half-life of 2 h is observed.

FIG. 53. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 5 incubated with 10 mM GSH at 25° C. The interval between each scan is ˜20 min. A half-life of 2.1 h is observed.

FIG. 54. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 6 at 25° C. The interval between each scan is ˜20 min. A half-life of 1.2 h was observed.

FIG. 55. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 6 incubated with 10 mM GSH at 25° C. The interval between each scan is ˜10 min. A half-life of 1.3 h was observed.

FIG. 56. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 7 at 25° C. The interval between each scan is ˜20 min. A half-life of 0.9 h was observed.

FIG. 57. UV-Vis spectra following an acetonitrile:PBS (1:1) solution of 7 incubated with 10 mM GSH at 25° C. The interval between each scan is ˜25 min. A half-life of 1.0 h was observed.

FIG. 58. a) Visible light induced E/Z isomerization of azo-BF₂ 10; b) UV-Vis spectra (6.46×10⁻⁵ M) of 10-E (purple) and 10-Z (orange) isomers in CH₂Cl₂; c) Isomerization cycles of 10 (2.12×10⁻⁵ M) in CH₂Cl₂ upon alternative irradiation using 600 and 430 nm light sources.

FIG. 59. a) Ball-stick drawing of the single crystal of 10; b) Head-to-head crystal packing through π-π interactions (black dashes); the hydrogen atoms have been omitted for clarity.

FIG. 60. The concentration dependent Z→E isomerization half-lives of the azo-BF₂ switch 10 in CD₂Cl₂.

FIG. 61. The in-situ switching of the isomerization half-life of switch 10, achieved by consecutively diluting and concentrating the NMR sample solution between 1.73 and 1.20 mM. The red error bars were calculated from the standard deviation of the average of three experiments.

FIG. 62. ¹H NMR spectrum of 16 in CDCl₃ at 294 K.

FIG. 63. ¹H NMR spectrum of 15 in CDCl₃ at 294 K.

FIG. 64. ¹H NMR spectrum of 14 in CDCl₃ at 294 K.

FIG. 65. ¹³C NMR spectrum of 14 in CDCl₃ at 294 K.

FIG. 66. ¹H NMR spectrum of 13 (contains a small percentage of the E isomer) in CD₂Cl₂ at 294 K.

FIG. 67. ¹³C NMR spectrum of 13 (contains a small percentage of the E isomer) in CD₂Cl₂ at 294 K.

FIG. 68. ¹H NMR spectrum of 12 in CD₂Cl₂ at 294 K.

FIG. 69. ¹³C NMR spectrum of 12 in CD₂Cl₂ at 294 K.

FIG. 70. ¹⁹F NMR spectrum of 12 in CD₂Cl₂ at 294 K.

FIG. 71. ¹H NMR spectrum of 10 (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K. The star indicates the methylene chloride peak.

FIG. 72. ¹³C NMR spectrum of 10 (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K.

FIG. 73. ¹⁹F NMR spectrum of 10 (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K.

FIG. 74. The 2D COSY of 10 in CD₂Cl₂ at 294 K.

FIG. 75. The 1D NOESY spectrum of 10 in CD₂Cl₂ at 294 K (H4 in 10 is irradiated).

FIG. 76. The 2D heteronuclear ¹H-¹⁹F NOESY spectrum of 10 in CD₂Cl₂ at 294 K, indicating the correlation between the fluorine signal and protons H1 and H9.

FIG. 77. ¹H NMR spectra of a) dark equilibrated 10, b) PPS at 600 nm, and c) PSS at 430 nm in CD₂Cl₂ at 294 K.

FIG. 78. Concentration-dependent UV-Vis absorption spectra of 10-trans in CH₂Cl₂ at room temperature.

FIG. 79. Linear fitting of the absorbance (2=535 nm) of 10-trans as a function of concentration in CH₂Cl₂ at room temperature.

FIG. 80. Concentration-dependent UV-Vis absorption spectra of 10-cis in CH₂Cl₂ at room temperature; the 10-cis isomer was obtained after irradiation at 600 nm for 1 min.

FIG. 81. Linear fitting of the absorbance (λ=501 nm) of 10-cis as a function of concentration in CH₂Cl₂ at room temperature.

FIG. 82. ¹H NMR spectra of compound 10 at different concentrations

FIG. 83. Partial ¹H NMR spectra of compound 10 at different concentrations showing the assigned proton H8 in both the trans and cis isomers. The arrow indicates a downfield shift of the proton signals with decreasing concentration.

FIG. 84. DOSY NMR spectrum of BF₂-azo 10 at a concentration of 0.15 mM.

FIG. 85. DOSY NMR spectrum of BF₂-azo 10 at a concentration of 0.53 mM.

FIG. 86. DOSY NMR spectrum of BF₂-azo 10 at a concentration of 1.51 mM.

FIG. 87. DOSY NMR spectrum of BF₂-azo 10 at a concentration of 8.26 mM.

FIG. 88. A plot of ln((A_(∞)-A)/(A_(∞)-A₀)) as a function of time showing the first-order decay of BF₂-azo 10 in CH₂Cl₂ at 294 K. A_(∞) and A₀ stand for the absorbance at the start and the end of thermal isomerization, respectively. The black line represents experimental data, while the red line represents the linear fitting curve. The fluctuation at the beginning of the relaxation curve may come from fluctuation in the solution after photoirradiation.

FIG. 89. A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 0.15 mM solution of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 90. A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 0.53 mM solution of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 91. A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 1.51 mM solution of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 92. A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 8.26 mM solution BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 93. Half-life switching experiment: A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 1.73 mM solution (initial concentration) of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 94. Half-life switching experiment: A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 1.20 mM solution (2^(nd) concentration) of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 95. Half-life switching experiment: A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 1.73 mM solution (3^(rd) concentration) of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 96. Half-life switching experiment: A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 1.20 mM solution (4^(th) concentration) of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 97. Half-life switching experiment: A plot of ln(C_(cis)) as a function of time showing the first-order decay of the 1.73 mM solution (5^(th) concentration) of BF₂-azo 10 in CD₂Cl₂ at 294 K.

FIG. 98. Linear fitting of the double logarithmic thermal isomerization rate constant (k) of BF₂-azo 10 as a function of its total concentration (C_(total)). The black squares represent the experimental data, while the red line represents the fitted curve.

FIG. 99. The 50% thermal ellipsoidal drawing of Azo-BF₂ 10 in the asymmetric cell with various amount of labeling using CPK colors.

DETAILED DESCRIPTION OF THE INVENTION

Photochromic Compounds of the Invention

Provided herein are compounds for use as photochromic molecular switches. In an embodiment, the compounds may be used in photopharmaceutical compounds as molecular switches that allow for selective spatiotemporal activation of pharmaceutical agents. As illustrated in FIG. 1a , one or more pharmaceutical agents (generically depicted as circles, stars, triangles and squares) may be bound to Compound 1 and activated by isomerization of Compound 1. Activation is schematically illustrated in FIG. 1a as a change from a filled shape (e.g., circle, star, triangle, square) to an unfilled shape or vice versa. Activation may, for example, include formation or breaking of one or more chemical bonds, severing of the molecular switch and pharmaceutical agent moieties, a change in configuration of the molecule to increase or decrease steric and/or electronic interactions (e.g., between the pharmaceutical agent and a protein binding pocket), a transfer of charge to/from/within the pharmaceutical agent, photon absorption or emission by the pharmaceutical agent, and/or any other process that changes the pharmaceutical agent from a first state to a second state, wherein the pharmaceutical agent in the first state does not alter a biological material or process, and wherein the pharmaceutical agent in the second state alters a biological material or process.

Also provided herein are methods of switching a molecular switch wherein electromagnetic radiation is applied to the molecular switch at a first wavelength effective to cause the trans→cis isomerization of a compound of Formula I or Formula II, a salt of Formula I or Formula II, or a photopharmaceutical compound derived from Formula I or Formula II; or applying electromagnetic radiation to the molecular switch at a second wavelength effective to cause the cis→trans isomerization of a compound of Formula I or Formula II, a salt of Formula I or Formula II, or a photopharmaceutical compound derived from Formula I or Formula II; or a combination thereof.

In one aspect, provided herein are compounds of Formula II:

or a salt thereof, wherein

N═N—R² can be oriented cis or trans to the tricycle;

R¹ is H, CN, CO₂H, CO₂(C₁₋₆-alkyl), C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl);

R² is C₆₋₁₉-aryl or C₃₋₁₄-heteroaryl, wherein the C₆₋₁₉-aryl or C₃₋₁₄-heteroaryl is independently substituted one or more times at the para and/or ortho position with C₁₋₆-alkyl, C₆₋₁₉-aryl, C₃₋₁₄-heteroalkyl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl); and

R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, H, C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, NHC(O)(C₁₋₆-alkyl) or a group corresponding to a small molecule pharmaceutical; or

R³ and R⁴, R⁴ and R⁵, R⁵ and R⁶ or R⁷ and R⁸ can, when taken together, form a fused aryl, fused heteroaryl, fused C₃₋₆-cycloalkyl, or fused heterocycle, wherein the fused aryl, fused heteroaryl, fused cycloalkyl, or fused heterocycle can be optionally substituted one or more times with C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl).

In another aspect, provided herein is a compound of the Formula I:

or a salt thereof, wherein

N═N—R² can be oriented cis or trans to the tricycle;

R¹ is H, CN, CO₂H, CO₂(C₁₋₆-alkyl), C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl);

R² is unsubstituted C₆₋₁₉-aryl or unsubstituted C₃₋₁₄-heteroaryl; and

R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, H, C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, NHC(O)(C₁₋₆-alkyl) or a group corresponding to a small molecule pharmaceutical; or

R³ and R⁴, R⁴ and R⁵, R⁵ and R⁶ or R⁷ and R⁸ can, when taken together, form a fused aryl, fused heteroaryl, fused C₃₋₆-cycloalkyl, or fused heterocycle, wherein the fused aryl, fused heteroaryl, fused cycloalkyl, or fused heterocycle can be optionally substituted one or more times with C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl).

In certain embodiments of Formula I and Formula II, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, H.

In certain other embodiments of Formula I and Formula II, R¹ is CN.

In certain embodiments of Formula I, R² is unsubstituted phenyl.

In certain embodiments of Formula II, R² is phenyl, wherein the phenyl is substituted at the para position. In certain other embodiments of Formula II, R² is phenyl, wherein the phenyl is substituted at the para position with N(CH₃)₂, OCH₃, piperazinyl, (N-methyl)piperazinyl, piperidinyl, morpholinyl or a group corresponding to a cholesterol molecule.

In some embodiments of Formula I and Formula II, N═N—R² is oriented cis to the tricycle. In other embodiments of Formula I and Formula II, N═N—R² is oriented trans to the tricycle.

In some embodiments of Formula II, the phenyl is substituted at the para position with OMe.

In other embodiments of Formula II, the phenyl is substituted at the para position with N(CH₃)₂.

In other embodiments of Formula II, the phenyl is substituted at the para position with piperazinyl.

In other embodiments of Formula II, the phenyl is substituted at the para position with piperidinyl.

In other embodiments of Formula II, the phenyl is substituted at the para position with (N-methyl)piperazinyl.

In other embodiments of Formula II, the phenyl is substituted at the para position with morpholinyl.

In other embodiments of Formula II, the phenyl is substituted at the para position with a group corresponding to a cholesterol molecule.

In certain embodiments of Formula I and Formula II, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are each, independently, a bond to a pharmaceutical agent.

In one aspect, provided herein is a photochromic molecular switch comprising at least one compound of Formula I or a salt thereof. In another aspect, provided herein is a photochromic molecular switch comprising at least one compound of Formula II or a salt thereof.

In certain embodiments, the molecular switch comprising the salt of Formula I or the salt of Formula II is reacted with a pharmaceutically acceptable salt to form a photopharmaceutical compound. In certain embodiments, the pharmaceutically acceptable salt is a salt of a small molecule pharmaceutical.

In one embodiment, the molecular switch comprising the compound of Formula I or the compound of Formula II or a photopharmaceutical compound thereof is stable in water or biological fluid. In another embodiment, the molecular switch comprising the compound of Formula I or the compound of Formula II or a photopharmaceutical compound thereof does not substantially hydrolyze in water or biological fluid.

Certain embodiments of Formula I and Formula II are shown in Table A and also are considered to be “compounds of the invention”.

TABLE A Compound No. Structure NMR and/or MS 1

¹H NMR (500 MHz, CD₂Cl₂) δ 8.42 (d, J = 9.0 Hz, 1H), 8.09 (d, J = 7.5 Hz, 1H), 7.91 (m, 2H), 7.63 (t, J = 8.0 Hz, 1H), 7.51 (m, 2H), 7.40 (m, 2H), 7.20 (m, 2H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 145.39, 144.61, 134.66, 134.29, 129.90, 129.84, 129.15, 128.95, 128.65, 127.21, 126.54, 126.03, 121.27, 119.63, 114.13 ppm; ¹⁹F NMR (282 MHz, CDCl₃) δ-150.66 (q, J = 28.3 Hz, 2F) ppm; GC-MS: calcd for C₁₇H₁₁BF₂N₄, 320.1; m/z (rel. inten.) 320 (15%, M⁺), 140 (29%), 113 (11%), 91 (25%), 77 (100%), 51(35%). 2

¹H NMR (500 MHz, CD₂Cl₂) δ 8.21 (d, J = 10.0 Hz, 1H), 7.81 (d, J = 10.0 Hz, 2H), 7.76 (t, J = 5.0 Hz, 1H), 7.72 (d, J = 10.0 Hz, 2H), 7.49 (t, J = 7.5 Hz, 1H), 7.34 (d, J = 10.0 Hz, 1H), 7.03 (d, J = 5.0 Hz, 2H), 3.91 (s, 3H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 161.30, 152.71, 144.76, 143.49, 139.17, 133.72, 129.60, 125.71, 125.04, 123.28, 118.34, 114.59, 114.20, 113.78, 112.91, 55.88 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ-145.95 (q, J = 19.7 Hz, 2F) ppm; GC-MS: calcd for C₁₈H₁₃BF₂N₄O, 350.1; m/z (rel. inten.) 350 (23%, M⁺), 144 (10%), 140(28%), 107 (100%), 91 (22%), 51 (32%). 3

¹H NMR (500 MHz, CD₂Cl₂) δ 8.36 (d, J = 10.0 Hz, 1H), 8.06 (d, J = 10.0 Hz, 1H), 7.88 (m, 2H), 7.60 (t, J = 7.5 Hz, 1H), 7.41 (d, J = 10.0 Hz, 1H), 7.21 (d, J = 10.0 Hz, 1H), 6.66 (d, J = 10.0 Hz, 1H), 6.56 (dd, J = 5.0 Hz, 1H), 3.95 (s, 3H), 3.90 (s, 3H) ppm, ¹³C NMR (126 MHz, CD₂Cl₂) δ 162.75, 161.85, 144.56, 143.48, 134.16, 133.67, 129.53, 125.73, 121.97, 119.26, 118.38, 114.17, 114.08, 105.14, 104.72, 99.24, 98.03, 56.46, 55.87 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ- 150.71 (q, J = 25.4 Hz, 2F) ppm; GC-MS: calcd for C₁₉H₁₅BF₂N₄O₂, 380.1; m/z (rel. inten.) 380 (24%, M⁺), 207 (70%), 137 (26%), 128 (30%), 73 (100%). 4

¹H NMR (500 MHz, CD₂Cl₂) δ 7.94 (d, J = 10.0 Hz, 1H), 7.86 (d, J = 10.0 Hz, 2H), 7.72 (d, J = 10.0 Hz, 1H), 7.66 (m, 2H), 7.36 (t, J = 10.0 Hz, 1H), 7.15 (d, J = 10.0 Hz, 1H), 6.80 (d, J = 10.0 Hz, 2H), 3.17 (s, 6H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.49, 141.25, 139.42, 132.86, 129.29, 128.25, 128.20, 128.15, 124.48. 124.36, 117.66, 117.64, 114.48, 114.01, 112.11, 40.30 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ-150.45 (q, J = 32.0 Hz, 2F) ppm; GC-MS: calcd for C₁₉H₁₆BF₂N₅, 363.1; m/z (rel. inten.) 363 (20%. M⁺), 281 (4%), 207 (11%), 134 (100%), 120 (49%), 65 (17%). 5

¹H NMR (500 MHz, CD₂Cl₂) δ 7.96 (d, J = 10.0 Hz, 1H), 7.82 (d, J = 7.5 Hz, 2H), 7.73 (d, J = 10.0 Hz, 1H), 7.67 (m, 2H), 7.37 (t, J = 10.0 Hz, 1H), 7.17 (d, J = 10.0 Hz, 1H), 6.95 (d, J = 10.0 Hz, 2H), 3.51 (t, J = 5.0 Hz, 4H), 1.72 (s, 6H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.69, 141.44, 138.81, 132.94, 129.32, 128.12, 128.04, 124.60, 124.43. 117.72, 117.31, 116.41, 114.47, 113.93, 113.74, 48.59, 25.78, 24.54 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ-150.11 (q, J = 18.8 Hz, 2F) ppm; GC-MS: calcd for C₂₂H₂₀BF₂N₅, 403.2; m/z (rel. inten.) 403 (14%, M⁺), 355 (10%), 281 (35%), 253 (12%), 207 (87%), 135 (31%), 73 (100%). 6

¹H NMR (500 MHz, CD₂Cl₂) δ 8.02 (d, J = 10.0 Hz, 1H), 7.81 (d, J = 5.0 Hz, 2H), 7.70 (m, 3H), 7.40 (t, J = 10.0 Hz, 1H), 7.21 (d, J = 10.0 Hz, 1H), 6.97 (d, J = 7.5 Hz, 2H), 3.48 (t, J = 5.0 Hz, 4H), 2.55 (t, J = 5.0 Hz, 4H), 2.33 (s, 3H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.46, 152.23, 141.96, 139.70, 139.35, 133.13, 129.39, 127.49, 127.39, 124.89, 124.58, 117.87, 114.40, 113.97, 113.70, 54.85, 47.22, 46.04 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ-149.31 (q, J = 18.8 Hz, 2F) ppm; GC-MS: calcd for C₂₂H₂₁BF₂N₆, 418.2; m/z (rel. inten.) 418 (20%, M⁺), 355 (12%), 281 (30%), 253 (13%), 207 (84%), 77 (100%). 7

¹H NMR (500 MHz, CD₂Cl₂) δ 8.06 (d, J = 5.0 Hz, 1H), 7.81 (d, J = 7.5 Hz, 2H), 7.73 (m, 3H), 7.42 (t, J = 7.5 Hz, 1H), 7.24 (d, J = 10.0 Hz, 1H), 6.97 (d, J = 10.0 Hz, 2H), 3.86 (t, J = 5.0 Hz, 4H), 3.41 (t, J= 5.0 Hz, 4H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.44, 142.31, 140.35, 139.30, 133.32, 129.52, 129.39, 127.09, 126.92. 125.09, 124.69, 117.97, 114.36, 114.07, 113.52, 66.65, 47.51 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ-148.73 (q, J = 18.8 Hz, 2F) ppm; GC-MS: calcd for C₂₁H₁₈BF₂N₅O, 405.2; m/z (rel. inten.) 405 (26%. M⁺), 355 (10%), 281 (32%), 253 (11%), 207 (83%), 135 (37%), 73 (100%). 8

¹H NMR (500 MHz CD₂Cl₂) δ: 7.81, 7.76, 7.62, 7.55, 7.47, 7.24, 7.03, 6.68, 4.33, 3.56, 1.75, 1.47 9

¹H NMR (500 MHz, CD₂Cl₂) δ = 8.41 (d, J = 8.9 Hz, 1H), 8.13 (dd, J = 6.8, 1.9 Hz, 2H), 7.91 (d, J = 7.9 Hz, 1H), 7.84 (m, 2H), 7.59 (t, J = 8 Hz, 1H), 7.53 (d, J = 8.6 Hz, 2H), 7.49 (d, J = 8.9 Hz, 1H), 5.37 (s, 1H), 4.86 (m, 1H), 2.50 (m, 1h), and 2.11-0.062 (cholesterol skeleton). ¹³C NMR (126 MHz, CD₂Cl₂) δ = 165.22, 152.68, 144.96, 140.02, 139.11, 134.41, 131.12, 130.25, 130.22, 129.77, 128.95, 126.77, 125.73, 122.88, 122.40, 118.87, 114.22, 75.00, 71.89, 65.69, 56.97, 56.39, 54.08, 53.86, 53.65, 53.43, 53.21, 50.35, 42.52, 40.01, 39.71, 38.40, 37.26, 36.87, 36.40, 36.04, 32.17, 32.11, 30.78, 28.42, 28.24, 28.07, 27.96, 24.47, 24.02, 22.77, 22.52, 21.27, 19.40, 19.37, 19.10, 18.72, 13.71, 11.84 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ = −150.66 (m, 2F); GC-MS: calcd. for C₄₄H₅₅BF₂N₄O₂ 732.33; m/z (rel. inten.) 732.2 (7.5%, M+). — Chol-H    Choleterol Photoisomerizable Group

A photoisomerizable group is one that changes from a first isomeric form to a second isomeric form upon exposure to electromagnetic radiation of different wavelengths, or upon a change in exposure from dark to light, or from light to dark. For example, in some embodiments, the photoisomerizable group is in a first isomeric form of a compound of Formula I or a first isomeric form of a compound of Formula II when exposed to electromagnetic radiation of a first wavelength, and is in a second isomeric form of a compound of Formula I or a second isomeric form of a compound of Formula II when exposed to electromagnetic radiation of a second wavelength.

The first wavelength and the second wavelength can differ from one another by from about 1 nm to about 600 nm or more, for example, from about 1 nm to about 10 nm, from about 10 nm to about 20 nm, from about 20 nm to about 50 nm, from about 50 nm to about 75 nm, from about 75 nm to about 100 nm, from about 100 nm to about 125 nm, from about 125 nm to about 150 nm, or from about 150 nm to about 200 nm, from about 200 nm to about 500 nm, from about 500 nm to about 600 nm, or more than 600 nm.

In other embodiments, the compound of Formula I or the compound of Formula II is in a first isomeric form when exposed to light of a wavelength λ₁, and is in a second isomeric form in the absence of light (e.g., in the absence of light, the compound undergoes spontaneous relaxation into the second isomeric form). In these embodiments, the first isomeric form of the compound of Formula I or the first isomeric form of a compound of Formula II is induced by exposure to light of wavelength λ₁, and the second isomeric form is induced by not exposing the compound to light (e.g., keeping the compound in darkness). In other embodiments, the compound of Formula I or the compound of Formula II is in a first isomeric form in the absence of light (e.g., when the compound is in the dark; and the compound is in a second isomeric form when exposed to light of a wavelength λ₁). In other embodiments, the compound of Formula I or the compound of Formula II is in a first isomeric form when exposed to light of a first wavelength λ₁, and then in a second isomeric form when exposed to light of a second wavelength λ₂.

For example, in some embodiments provided herein, the compound of Formula I or the compound of Formula II is in a trans configuration in the absence of light, or when exposed to light of a first wavelength; and the photoisomerizable group is in a cis configuration when exposed to light of a second wavelength that is different from the first wavelength.

In some embodiments, the wavelength of light that effects a change in the compound of Formula I or in the compound of Formula II from a first isomeric form to a second isomeric form ranges from 400 nanometers to 1000 nanometers.

In other embodiments, the wavelength of light that effects a change in the compound of Formula I or in the compound of Formula II from a first isomeric form to a second isomeric form ranges from 450 nanometers to 850 nanometers.

In other embodiments, the wavelength of light that effects a change in the compound of Formula I or in the compound of Formula II from a first isomeric form to a second isomeric form ranges from 710 nanometers to 760 nanometers.

The difference between the first wavelength and the second wavelength can range from about 1 nm to about 600 nm or more, as described above. Of course, where the synthetic light regulator is switched from light to darkness, the difference in wavelength is from the wavelength λ₂ to essentially zero.

Methods of Using Molecular Switches

The select application of electromagnetic radiation to the compounds of Formula I or the compounds of Formula II or photopharmaceutical compounds comprising a Formula I or Formula II moiety can be used to transform a compound from a trans azo isomeric form to a cis azo isomeric form or vice versa. In this way, the compounds can be used as a molecular switch, with the electromagnetic radiation serving as a switching means.

In some embodiments, the compounds of Formula I and Formula II or photopharmaceutical compounds thereof respond to electromagnetic radiation that is generated by an infrared light source and/or a visible light source. Therefore, in some of these embodiments, the compounds of Formula I or the compounds of Formula II respond to a first wavelength λ₁ ranging from 400 nanometers to 1 millimeter to convert the trans azo isomer to the cis azo isomer; and a second wavelength λ₂ ranging from 400 nanometers to 1 millimeter causes the cis azo isomer to convert back to the trans azo isomer.

For example, in certain embodiments, the compounds of Formula I or the compounds of Formula II or photopharmaceutical compounds thereof respond to a first wavelength λ₁ ranging from 700 nanometers to 1000 nanometers to convert the trans azo isomer to the cis azo isomer; and to a second wavelength λ₂ ranging from 700 nanometers to 1000 nanometers. Additionally, in certain other embodiments, the compounds of Formula I or the compounds of Formula II or photopharmaceutical compounds thereof respond to a first wavelength λ₁ ranging from 450 nanometers to 850 nanometers to convert the trans azo isomer to the cis azo isomer; and a second wavelength λ₂ ranging from 450 nanometers to 850 nanometers causes the cis azo isomer to convert back to the trans azo isomer.

In some other embodiments, the compounds of Formula I or the compounds of Formula II or photopharmaceutical compounds thereof respond to electromagnetic radiation that is generated by a visible light source. Therefore, in some of these embodiments, the compounds of Formula I or the compounds of Formula II or photopharmaceutical compounds thereof respond to a first wavelength λ₁ ranging from 570 to 750 nanometers to convert the trans azo isomer to the cis azo isomer; and a second wavelength λ₂ ranging from 450 to 495 nanometers causes the cis azo isomer to convert back to the trans azo isomer.

The skilled artisan will appreciate that the photochromic materials described herein need not be limited to those responsive to the exemplary wavelength ranges described herein.

Definitions

In this disclosure, “comprises,” “comprising,” “containing,” and “having” and the like can have the meaning ascribed to them in U.S. Patent law and can mean “includes,” “including,” and the like; “consisting essentially of” or “consists essentially” likewise has the meaning ascribed in U.S. Patent law and the term is open-ended, allowing for the presence of more than that which is recited so long as basic or novel characteristics of that which is recited is not changed by the presence of more than that which is recited, but excludes prior art embodiments.

Unless otherwise specified, “a” or “an” means “one or more”.

The term “electromagnetic radiation” as used herein, refers to a form of energy exhibiting wave like behavior and having both electric and magnetic field components, which oscillate in phase perpendicular to each other as well as perpendicular to the direction of energy propagation. Electromagnetic radiation is classified according to the frequency of its wave. In order of increasing frequency (f) and decreasing wavelength (k), electromagnetic radiation includes: radio waves (3 Hz≤f≤300 MHz; 1 m≤λ≤100,000 Km), microwaves (300 MHz≤f≤300 GHz; 1 mm≤λ≤1 m), infrared radiation (300 GHz≤f≤400 THz; 750 nm≤λ≤1 m), visible light (400 THz≤f≤770 THz; 400 nm≤λ≤1 m), ultraviolet radiation (750 THz≤f≤30 PHz; 10 nm≤λ≤400 nm), X-rays (300 PHz≤f≤30 EHz; 0.01 nm≤λ≤10 nm), and gamma rays (f≥15 EHz; λ≤0.02 nm). Hence, the term “electromagnetic radiation”, as used herein, denotes photons, particularly in the visible range and/or in the infrared region.

The term “photochromism”, as used herein, indicates a photoinduced change in color. Herein, the interconversion usually occurs between two colored states. A photochromic transformation is always accompanied by profound absorbance changes in the visible region. In fact, visible absorption spectroscopy is the most convenient analytical method to study these processes.

The term “photostationary state”, as used herein, indicates a state where no further changes in the UV/Vis spectra are observed. For example, application of additional electromagnetic energy to a sample or molecule in a photo stationary state does not produce a change in absorption or emission spectra.

As used herein, the term “molecular switch” refers to a molecule that generates a change in state in response to a signal. In one aspect, a molecular switch is capable of switching from at least one state to at least one other state in response to the signal.

As used herein, the expression “a group corresponding to” an indicated species expressly includes a radical (including a monovalent, divalent and trivalent radical), for example an aromatic radical or heterocyclic aromatic radical, of the species or group of species provided in a covalently bonded configuration.

As used herein, the term “group” may refer to a functional group of a chemical compound. Groups of the present compounds refer to an atom or a collection of atoms that are a part of the compound. Groups of the present invention may be attached to other atoms of the compound via one or more covalent bonds. Groups may also be characterized with respect to their valence state. The present invention includes groups characterized as monovalent, divalent, trivalent, etc.

As used herein, the term “substituted” refers to a compound wherein a hydrogen is replaced by another functional group.

As used herein, the term “a small molecule pharmaceutical” refers to a compound, typically an organic compound, with a molecular weight of 900 Daltons or less, or 500 Daltons or less, where the compound has a therapeutic functionality or a diagnostic functionality. In some embodiments, a small molecule pharmaceutical is a compound that is, within the scope of sound medical judgment, suitable for use in contact with the tissues of humans and lower animals without undue toxicity, irritation, allergic response and the like, and that is commensurate with a reasonable benefit/risk ratio.

As used herein, the term “photopharmaceutical compound” refers to a molecule comprising a molecular switch moiety and a pharmaceutical compound moiety, where the moieties may be covalently, ionically or electrostatically bonded or attracted. In some embodiments, a photopharmaceutical compound is synthesized by reacting a salt of a molecular switch with a pharmaceutically acceptable salt. Salts of molecular switches are compounds wherein the parent compound is modified by converting an existing acid or base moiety to its salt form. Examples of salts include, but are not limited to, mineral or organic acid salts of basic residues such as amines; alkali or organic salts of acidic residues such as carboxylic acids; and the like. The salts can be synthesized from the parent compound which contains a basic or acidic moiety by conventional chemical methods. Generally, such salts can be prepared by reacting the free acid or base forms of these compounds with a stoichiometric amount of the appropriate base or acid in water or in an organic solvent, or in a mixture of the two.

The term “pharmaceutically acceptable salt” refers to those salts of pharmaceutical compounds which are, within the scope of sound medical judgment, suitable for use in contact with the tissues of humans and lower animals without undue toxicity, irritation, allergic response and the like, and are commensurate with a reasonable benefit/risk ratio. Additionally, “pharmaceutically acceptable salts” refers to compounds wherein the parent compound is modified by converting an existing acid or base moiety to its salt form. Examples of pharmaceutically acceptable salts include, but are not limited to, mineral or organic acid salts of basic residues such as amines; alkali or organic salts of acidic residues such as carboxylic acids; and the like. The pharmaceutically acceptable salts include the conventional non-toxic salts of the parent compound formed, for example, from non-toxic inorganic or organic acids. The pharmaceutically acceptable salts can be synthesized from the parent compound which contains a basic or acidic moiety by conventional chemical methods. Generally, such salts can be prepared by reacting the free acid or base forms of these compounds with a stoichiometric amount of the appropriate base or acid in water or in an organic solvent, or in a mixture of the two; generally, nonaqueous media like ether, ethyl acetate, ethanol, isopropanol, or acetonitrile are preferred. Lists of suitable salts are found in Remington's Pharmaceutical Sciences, 17^(th) ed., Mack Publishing Company, Easton, Pa., 1985, p. 1418 and Journal of Pharmaceutical Science, 66, 2 (1977), each of which is incorporated herein by reference in its entirety.

As used herein, the term “tissue target” or “target of a tissue” refers to biomolecules or fragments thereof including, but not limited to, hormones, amino acids, peptides, peptidomimetics, proteins, nucleosides, nucleotides, nucleic acids, enzymes, carbohydrates, glycomimetics, lipids, albumins, mono- and polyclonal antibodies, receptors, inclusion compounds such as cyclodextrins, and receptor binding molecules.

As used herein, the term “alkyl” refers to a fully saturated branched or unbranched hydrocarbon moiety. Preferably the alkyl comprises 1 to 20 carbon atoms, more preferably 1 to 16 carbon atoms, 1 to 10 carbon atoms, 1 to 7 carbon atoms, 1 to 6 carbons, 1 to 4 carbons, or 1 to 3 carbon atoms. Representative examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl, 3-methylhexyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, n-heptyl, n-octyl, n-nonyl, n-decyl and the like. Furthermore, the expression “C_(x)-C_(y)-alkyl”, wherein x is 1-5 and y is 2-10 indicates a particular alkyl group (straight- or branched-chain) of a particular range of carbons. For example, the expression C₁-C₄-alkyl includes, but is not limited to, methyl, ethyl, propyl, butyl, isopropyl, tert-butyl and isobutyl.

As used herein, the term “cycloalkyl” refers to saturated or unsaturated monocyclic, bicyclic or tricyclic hydrocarbon groups of 3-12 carbon atoms, preferably 3-9, or 3-7 carbon atoms. Exemplary monocyclic hydrocarbon groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclopentenyl, cyclohexyl and cyclohexenyl and the like. Exemplary bicyclic hydrocarbon groups include bornyl, indyl, hexahydroindyl, tetrahydronaphthyl, decahydronaphthyl, bicyclo[2.1.1]hexyl, bicyclo[2.2.1]heptyl, bicyclo[2.2.1]heptenyl, 6,6-dimethylbicyclo[3.1.1]heptyl, 2,6,6-trimethylbicyclo[3.1.1]heptyl, bicyclo[2.2.2]octyl and the like. Exemplary tricyclic hydrocarbon groups include adamantyl and the like.

The term “heteroalkyl,” by itself or in combination with another term, means, unless otherwise stated, a stable straight or branched chain, or combinations thereof, consisting of the stated number of carbon atoms and from one to five heteroatoms, more preferably from one to three heteroatoms, selected from the group consisting of O, N, Si and S, and wherein the nitrogen and sulfur atoms may optionally be oxidized and the nitrogen heteroatom may optionally be quaternized. The heteroalkyl group is attached to the remainder of the molecule through a carbon atom or a heteroatom.

The term “aryl” includes aromatic monocyclic or multicyclic (e.g., tricyclic, bicyclic), hydrocarbon ring systems consisting only of hydrogen and carbon and containing from six to nineteen carbon atoms, or six to ten carbon atoms, where the ring systems may be partially saturated. Aryl groups include, but are not limited to, groups such as phenyl, tolyl, xylyl, anthryl, naphthyl and phenanthryl. Aryl groups can also be fused or bridged with alicyclic or heterocyclic rings which are not aromatic so as to form a polycycle (e.g., tetralin).

The term “heteroaryl,” as used herein, represents a stable monocyclic or bicyclic ring of up to 7 atoms in each ring, wherein at least one ring is aromatic and contains from 1 to 4 heteroatoms selected from the group consisting of O, N and S. Heteroaryl groups within the scope of this definition include but are not limited to: acridinyl, carbazolyl, cinnolinyl, quinoxalinyl, pyrazolyl, indolyl, benzotriazolyl, furanyl, thienyl, benzothienyl, benzofuranyl, quinolinyl, isoquinolinyl, oxazolyl, isoxazolyl, indolyl, pyrazinyl, pyridazinyl, pyridinyl, pyrimidinyl, pyrrolyl, tetrahydroquinoline. As with the definition of heterocycle below, “heteroaryl” is also understood to include the N-oxide derivative of any nitrogen-containing heteroaryl. In cases where the heteroaryl substituent is bicyclic and one ring is non-aromatic or contains no heteroatoms, it is understood that attachment is via the aromatic ring or via the heteroatom containing ring, respectively.

The term “heterocycle” or “heterocycyl” refers to five-member to ten-member, fully saturated or partially unsaturated nonaromatic heterocylic groups containing at least one heteroatom such as 0, S or N. The most frequent examples are piperidinyl, morpholinyl, piperazinyl, pyrrolidinyl or pyrazinyl. Attachment of a heterocycyl substituent can occur via a carbon atom or via a heteroatom.

Moreover, the alkyl, alkenyl, cycloalkyl, cycloalkenyl, alkoxy, aryl, heteroaryl, and heterocycle groups described above can be “unsubstituted” or “substituted.”

The term “substituted” is intended to describe moieties having substituents replacing a hydrogen on one or more atoms, e.g. C, O or N, of a molecule. Such substituents can independently include, for example, one or more of the following: straight or branched alkyl (preferably C₁-C₅), cycloalkyl (preferably C₃-C₈), alkoxy (preferably C₁-C₆), thioalkyl (preferably C₁-C₆), alkenyl (preferably C₂-C₆), alkynyl (preferably C₂-C₆), heterocyclic, carbocyclic, aryl (e.g., phenyl), aryloxy (e.g., phenoxy), aralkyl (e.g., benzyl), aryloxyalkyl (e.g., phenyloxyalkyl), arylacetamidoyl, alkylaryl, heteroaralkyl, alkylcarbonyl and arylcarbonyl or other such acyl group, heteroarylcarbonyl, or heteroaryl group, (CR′R″)₀₋₃NR′R″ (e.g., —NH₂), (CR′R″)₀₋₃CN (e.g., —CN), —NO₂, halogen (e.g., —F, —Cl, —Br, or —I), (CR′R″)₀₋₃C(halogen)₃ (e.g., —CF), (CR′R″)₀₋₃CH(halogen)₂, (CR′R″)₀₋₃CH₂(halogen), (CR′R″)₀₋₃CONR′R″, (CR′R″)₀₋₃(CNH)NR′R″, (CR′R″)₀₋₃S(O)₁₋₂NR′R″, (CR′R″)₀₋₃CHO, (CR′R″)₀₋₃O(CR′R″)₀₋₃H, (CR′R″)₀₋₃S(O)₀₋₃R′ (e.g., —SO₃H, —OSO₃H), (CR′R″)₀₋₃O(CR′R″)₀₋₃H (e.g., —CH₂OCH₃ and —OCH₃), (CR′R″)₀₋₃S(CR′R″)₀₋₃H (e.g., —SH and —SCH₃), (CR′R″)₀₋₃OH (e.g., —OH), (CR′R″)₀₋₃COR′, (CR′R″)₀₋₃ (substituted or unsubstituted phenyl), (CR′R″)₀₋₃(C₃-C₈ cycloalkyl), (CR′R″)₀₋₃CO₂R′ (e.g., —CO₂H), or (CR′R″)₀₋₃OR′ group, or the side chain of any naturally occurring amino acid; wherein R′ and R″ are each independently hydrogen, a C₁-C₅ alkyl, C₂-C₅ alkenyl, C₂-C₅ alkynyl, or aryl group.

As is customary and well known in the art, hydrogen atoms are not always explicitly shown on chemical structures, for example, hydrogen atoms bonded to the carbon atoms of aromatic, heteroaromatic, and alicyclic rings are not always explicitly shown.

The description of the disclosure herein should be construed in congruity with the laws and principals of chemical bonding. For example, it may be necessary to remove a hydrogen atom in order accommodate a substituent at any given location. Furthermore, it is to be understood that definitions of the variables (i.e., “R groups”), as well as the bond locations of the generic formula of the invention (i.e., Formula I or Formula II), will be consistent with the laws of chemical bonding known in the art. It is also to be understood that all of the compounds of the invention described above will further include bonds between adjacent atoms and/or hydrogens as required to satisfy the valence of each atom. That is, bonds and/or hydrogen atoms are added to provide the following number of total bonds to each of the following types of atoms: carbon: four bonds; nitrogen: three bonds; oxygen: two bonds; and sulfur: two-six bonds.

The compounds of this invention may include asymmetric carbon atoms. It is to be understood accordingly that the isomers arising from such asymmetry (e.g., all enantiomers, stereoisomers, rotamers, tautomers, diastereomers, or racemates) are included within the scope of this invention. Such isomers can be obtained in substantially pure form by classical separation techniques and by stereochemically controlled synthesis. Furthermore, the structures and other compounds and moieties discussed in this application also include all tautomers thereof. Compounds described herein may be obtained through art recognized synthesis strategies.

It will also be noted that the substituents of some of the compounds of this invention include isomeric cyclic structures. It is to be understood accordingly that constitutional isomers of particular substituents are included within the scope of this invention, unless indicated otherwise. For example, the term “tetrazole” includes tetrazole, 2H-tetrazole, 3H-tetrazole, 4H-tetrazole and 5H-tetrazole.

EXEMPLIFICATION

The invention is further illustrated by the following examples, which should not be construed as further limiting. The practice of the present invention will employ, unless otherwise indicated, conventional techniques of synthetic and physical organic chemistry as well as computational chemistry, which are within the skill of the art.

Example 1: Synthesis of Compound 1 1.1 Synthesis of Hydrozone 1c

1.7 mL of HBF₄ (48%) was added dropwise to a mixture of aniline (0.31 mL, 3.4 mmol) in 1 mL water. After stirring in an ice-bath for 40 min, a precooled solution of NaNO₂ (1 equiv, 0.235 g, 3.4 mmol) was then added dropwise over a period of 15 min. The white diazonium salt was collected by filtration after 90 min and added to a suspension of 2-quinolylacetonitrile (0.8 equiv, 0.456 g, 2.7 mmol) and sodium acetate (3.2 equiv, 0.890 g, 10.8 mmol) in a cooled and well stirred 30 mL ethanol/water (2:1) mixture. The resulting reaction mixture was left to stir overnight at RT. The precipitated compound was then collected by filtration and dried over air. The crude product was purified by silica gel column chromatography (hexane/ethyl acetate 15:1) to give hydrazone 1c as a bright yellow powder (0.550 g, 75%). Mp: 159.8-160.4° C.; ¹H NMR (500 MHz, CDCl₃) δ 16.10 (s, 1H), 8.31 (d, J=8.5 Hz, 1H), 8.05 (d, J=8.5 Hz, 1H), 7.86 (m, 2H), 7.81 (t, J=8.5 Hz, 1H), 7.63 (t, J=8.0 Hz, 1H), 7.44 (m, 4H), 7.15 (t, J=9.5 Hz, 1H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 152.31, 145.82, 142.47, 138.00, 131.05, 129.94, 129.81, 128.32, 128.10, 127.88, 127.05, 124.70, 119.81, 118.09, 115.50 ppm; GC-MS: calcd for C₁₇H₁₂N₄, 272.1; m/z (rel. inten.) 272 (56%, M⁺), 167 (21%), 140 (34%), 105 (15%), 77 (100%), 51 (15%).

1.2 Synthesis of Compound 1

N,N-Diisopropylethylamine (DIPEA, 7 equiv, 0.22 mL, 1.3 mmol) was added to a solution of hydrazone 1c (0.050 g, 0.18 mmol) in dry methylene chloride at room temperature. After 2 hours, boron trifluoride diethyl ether complex (10 equiv, 0.23 mL, 1.8 mmol) was added dropwise. The reaction mixture was stirred at room temperature overnight. The reaction mixture was quenched with water and extracted by methylene chloride. The organic layer was washed two times with 10 mL water, 10 mL saturated sodium bicarbonate solution and dried over magnesium sulfate. After solvent concentration, the crude product was subjected to silica gel column chromatography (methylene chloride/hexane 2:1) to give compound 1 as a dark red solid (40 mg, 68%). The compound starts to decompose at 177.5° C. before reaching its mp; (¹H NMR 500 MHz, CD₂Cl₂) δ 8.42 (d, J=9.0 Hz, 1H), 8.09 (d, J=7.5 Hz, 1H), 7.91 (m, 2H), 7.63 (t, J=8.0 Hz, 1H), 7.51 (m, 2H), 7.40 (m, 2H), 7.20 (m, 2H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 145.39, 144.61, 134.66, 134.29, 129.90, 129.84, 129.15, 128.95, 128.65, 127.21, 126.54, 126.03, 121.27, 119.63, 114.13 ppm; ¹⁹F NMR (282 MHz, CDCl₃) δ −150.66 (q, J=28.3 Hz, 2F) ppm; GC-MS: calcd for C₁₇H₁₁BF₂N₄, 320.1; m/z (rel. inten.) 320 (15%, M⁺), 140 (29%), 113 (11%), 91 (25%), 77 (100%), 51 (35%).

The compound (1d) was formed as the minor product during the same reaction of compound 1. After purification by silica gel column chromatography (hexane/ethyl acetate 5:1), 1d was collected as a bright yellow solid with 10% yield. When the reaction was carried out at 60° C., 1 was obtained as the major product (40% yield). mp 229.2-229.6° C.; ¹H NMR (500 MHz, CDCl3) δ 8.87 (d, J=5.0 Hz, 1H), 8.55 (d, J=4.5 Hz, 1H), 7.93 (m, 5H), 7.75 (t, J=7.5 Hz, 1H), 7.46 (m, 2H), 7.32 (t, J=7.3 Hz, 1H) ppm; ¹³C NMR (126 MHz, CDCl3) δ 144.04, 142.65, 133.96, 129.27, 128.87, 127.94, 127.36, 124.05, 123.99, 123.92, 121.09, 121.07, 127.05, 117.78, 116.17 ppm; ¹⁹F NMR (282 MHz, CDCl3) δ −124.17 (q, J=28.3 Hz, 2F) ppm; GC-MS: calcd for C₁₇H₁₁BF₂N₄, 320.1; m/z (rel. inten.) 320 (4%, M⁺), 154 (2%), 128 (5%), 113 (9%), 101 (8%), 91 (17%), 77 (100%).

Compounds of Formula II can be prepared in accordance with the outlined synthesis of Compound I, but starting from aryl-substituted analogues of aniline 1a (e.g., Compound 2 can be prepared starting from p-methoxyaniline; Compound 4 can be prepared starting from p-aminodimethylamine). Subsequently, the substituted aniline can be reduced to a tetrafluoroborate diazonium salt analogous to 1b. Further reaction of 1b-analogues afford aryl-substituted analogues of hydrozone 1c, which can be used to prepare BF₂-coordinated compounds of Formula II, as outlined in Example 1.2.

Example 2: Photoisomerism of Compound 1

The photoisomerization of Compound 1 was studied extensively by UV/vis (FIG. 1) spectroscopy. When stored in the dark, Compound 1 adopts its thermodynamically stable trans form that has an absorption maximum (λ_(max)) at 530 nm (ε=8026 M⁻¹ cm⁻¹). Upon irradiation at 570 nm, the cis form (λ_(max)=480 nm; £=7792 M⁻¹ cm⁻¹) becomes dominant, accompanied by a sharp color change of the solution from bright purple to light orange. The process is also accompanied by changes in the intensity of bands at higher energies (λ_(max)=340 and 264 nm). Irradiation at 450 nm drives the system back to its trans form. The isosbestic points (λ_(max)=499, 399, 330, and 257 nm) in the UV/vis spectra demonstrate that only two species are exchanging during the isomerization process (FIG. 1b ). The trans/cis isomerization can be activated solely by the use of visible light and there is no need for UV light. Furthermore, as shown in FIG. 1c , the system shows very good reversibility as no degradation of Compound 1 was observed during the entire photoisomerization studies that lasted for more than a month.

Example 3: The Calculated (B3LPY/6-311++G**)

To understand the effect of Lewis acid coordination on the photophysical properties of azo compounds, computational modeling of the trans and cis isomers of Compound 1 was conducted. Structures were optimized by density functional theory (DFT) using the B3LYP hybrid functional and the 6-311++G** basis set, as implemented in Jaguar; this combination of method and basis set is appropriate for such systems. The optimized structure of trans-Compound 1 matches well with its crystal structure (FIGS. 4 and 5). Calculations of the UV/Vis spectra of the optimized structures were carried out in ADF using time-dependent DFT (TDDFT) using the B3LYP functional and a triple-ζ basis with two added polarization functions (TZ2P). These calculations were also successful in predicting the UV/vis spectra of the cis and trans isomers of Compound 1 (FIGS. 8 and 9), and show that the absorption bands in the visible range (FIG. 3, Table C and FIGS. 14-18S37-S41) stem from 7-nonbonding to π*-antibonding transitions (HOMO→LUMO).

The calculations predict correctly the separation between the cis (λ_(max)=482 nm) and trans bands (λ_(max)=510 nm). Another set of π_(nb)-π* transitions (HOMO to LUMO+1) is also predicted at a higher energy level (λ_(max)=376 and 353 nm for trans and cis, respectively) where a smaller band is clearly visible in the UV/vis spectrum (FIG. 2a ), whereas the n-π* transition is, as predicted, at an even shorter wavelength (λ_(max)=338 nm for trans).

Relative to azobenzenes, binding to BF₂ drastically lowers the energy of the n-electrons, while additional conjugation in the N—C—C—N—N skeleton provides a higher energy π_(nb) molecular orbital which serves as the HOMO (FIGS. 14-18S37-S41). This in turns leads to the strong absorption band in the visible range that enables the manipulation of Compound 1 using only visible light. The BF₂ group is not unique in promoting this effect. As a proof of concept, the BF₂ group was replaced with Na⁺ (in silico), which also resulted in red-shifted UV/vis absorption spectra (FIGS. 10 and 11). Intriguingly, the trans and cis isomers have a λ_(max) of 466 and 472 nm, respectively. Moreover, the calculations predict that replacing the CN group with π-electron donating groups (FIGS. 12 and 13) and/or substituting the phenyl ring with such groups (based on the HOMO in FIGS. 6-7) will lead to a red shift in the absorption band, opening the door for further manipulations of the photophysical properties of the azo compound.

NMR Photoisomerization Studies

Deoxygenated CD₂Cl₂ solutions of Compound 1 (8.8 mM) in quartz NMR tubes were used for all the ¹H NMR measurements. Irradiation of the samples was conducted with sufficient stirring. The lamp intensity at 450 nm was determined by chemical actinometry using potassium ferrioxalate and ferrozine while the intensity at 570 nm was determined by a Thorlabs optical power meter. The background thermal cis→trans isomerization was measured using ¹H NMR spectroscopy and no observable trans isomer was formed during the time period used in the irradiation experiments.

Photoisomerization Quantum Yields

The experimental procedure was adapted from Bandara et al. (J. Org. Chem. 2010, 75, 4817). After measuring the light intensities as mentioned above, the photoisomerization quantum yields were determined by following the change in isomer ratio after a certain period of irradiation time.

Kinetic Studies

A 0.2 mM deoxygenated CH₂Cl₂ solution of Compound 1 in a quartz cuvette was irradiated at 570 nm until its photostationary state was reached (no further changes in the UV/Vis spectra were observed). The thermal isomerization process was monitored by measuring the change in absorption intensity at 480 nm as a function of time (at 1 min intervals). The half-life (t_(1/2)) of the cis*trans isomerization was calculated to be 12.5 h, which is the average of three measurements conducted at the same conditions.

X-Ray Crystallography

Data were collected using a Bruker CCD (charge coupled device) based diffractometer equipped with an Oxford Cryostream low-temperature apparatus operating at 173 K. Data were measured using omega and phi scans of 1° per frame for 30 s. The total number of images was based on results from the program COSMO where redundancy was expected to be 4.0 and completeness to 0.83 Å to 100%. Cell parameters were retrieved using APEX II software and refined using SAINT on all observed reflections. Data reduction was performed using the SAINT software which corrects for Lp. Scaling and absorption corrections were applied using SADABS multi-scan technique, supplied by George Sheldrick. The structures are solved by the direct method using the SHELXS-97 and refined by least squares method on F², SHELXL-97, which are incorporated in SHELXTL-PC V 6.10. The structure of Compound 1 was solved in the space group P2₁/c (#14). The aromatic ring was found to be disordered and was modeled at 50% in each position. The structure of Compound 1d was solved in the space group C2/c (#15). All non-hydrogen atoms were refined anisotropically. Hydrogens were calculated by geometrical methods and refined as a riding model. Crystals used for the diffraction studies showed no decomposition during data collection.

DFT Calculations

Computational Methods

All reported DFT calculations were carried out, without any symmetry constraints, using the robust hybrid B3LYP functional and the 6-311G**++basis set, as implemented in the Jaguar suite of programs. Vibrational frequencies were calculated using analytic second derivatives, and all structures were confirmed as minima by the absence of imaginary frequencies. The allowed transitions in the UV spectra for the B3LYP optimized structures were carried out using TDDFT (B3LYP/TZ2P), as implemented in the ADF suite of programs.

Example 4: Near Infrared Light Activated Azo-BF₂ Switches

Light penetration through tissue is primarily regulated by the absorptions of hemoglobin and water,¹ which limits its “therapeutic window” to the 600-1200 nm range. In principle, the more red-shifted the wavelength the deeper the penetration, hence, near infrared (NIR) is far better than red light in this aspect.² One way of using this property of light is to couple it with photochromic³ compounds that are capable of reversibly modulating biological processes. This is the main objective of the fields of photopharmacology⁴ and opto(chemical)genetics.⁵ Azo compounds⁶ are the most commonly used light activated switches' in these research areas because of their efficient trans/cis photoisomerization. However, this process generally relies on UV light, which is not biocompatible.⁸ Consequently, there has been intense activity in the field in trying to shift the activation wavelength of these photochromic compounds to the visible region,⁹ and beyond. The burgeoning activity has led to the development of a number of visible light activated azo compounds, through appropriate derivatization,¹⁰ or the use of metal to ligand charge transfer,¹¹ among other approaches.¹² This activity has recently paid off with the seminal work by Woolley et al. describing the in vivo activation of an azo compound using red light (635 nm).^(10c) Despite these recent advances there is still a need to develop more efficient systems, and push the activation wavelength of the azo compounds beyond the red region, in order to gain access to deeper tissues.¹³ In this context Qian et al. have recently showed¹⁴ how NIR activated upconversion nanoparticles¹⁵ can be used in manipulating azo compounds; however, this was accomplished using the NIR light indirectly, as the azo switch was still modulated using the UV light emitting from the excited nanoparticles. Furthermore the incorporation of nanoparticles complicates the system, and leads to low isomerization efficiencies, reducing the practicality of this approach in photopharmacology and opto(chemical)genetics.

The inventors discovered¹⁶ that the coordination of BF₂ with an azo group's nitrogen lone-pair leads to a reversal of the positions of n-π* and π-π* transition energy levels. This property enabled switching of the azo-compounds using visible (i.e., blue and green) light. DFT calculations predicted that increasing the electron-density in the system could further red-shift the absorption bands, and hence activation wavelength of the system. A series of azo-BF₂ complexes having electron donating para- and ortho-substituents (Scheme 1) were designed.¹⁷

This example reports how such substituents shift the azo-BF₂ absorption bands to the red and even NIR region, thus enabling direct and efficient isomerization of an azo-compound using NIR light. These systems are also stable to glutathione reduction, and have relatively long lived half lives in aqueous solutions.

As predicted by DFT calculations the introduction of a methoxy group para to the azo linkage (Scheme 2) leads to a bathochromic shift in the π-π* band of 2 (λ_(max)=594 nm, £=15,998 M⁻¹ cm⁻¹).

The λ_(max) of the cis photostationary state (PSS; irradiation at 630 nm) of compound 2 is red-shifted by 40 nm (FIG. 19a ), while the trans PSS (irradiation at 490 nm) is red-shifted by 55 nm compared with the parent complex 1.¹⁶ The switching process of 2 was accompanied with a strong color change between cobalt blue and poppy red. High photoconversion ratios (PSS490=92% trans, PSS630=96% cis) and a 93% trans isomer ratio under dark were determined for 2 using ¹H NMR spectroscopic analysis (FIGS. 27 and 26). There are several spectral features of 2 that set it apart from the parent compound; instead of a sharp band, the π-π* band of the trans-dominant state exhibits a well resolved vibrational fine structure, with the highest-intensity peak observed at λ_(max)=594 nm. The appearance of the sub-bands can be attributed to the intensified vibrational transitions caused by the electron-donating para-substituent.¹⁸ In addition, the isomerization process in 2 is more efficient than 1 based on its quantum yields (Φ_(trans→cis)=71% and Φ_(cis→trans)=95%) This efficiency enhancement results from a better separation of the cis and trans states' π-π* bands, which leads to less overlap of their irradiation windows compared to the parent azo complex 1. The cis isomer of 2 has a half-life of 10.4 h at 294 K in deoxygenated methylene chloride (FIG. 48), compared to 25 min in regular solvent (not deoxygenated).^(16, 1)

It was recently shown^(10b) that ortho-tetrafluoroazobenzene has a slightly blue shifted π-π* absorption when compared to its parent azobenzene, and that the substitution of the ortho positions with methoxy groups^(10a) leads to the separation of the n-π* orbital of the cis and trans isomers and a red-shift of the π-π* band in the latter. In order to study the ortho-substitution effect, complex 3 (Scheme 3) was prepared. The extra ortho-methoxy group in 3 causes a 20 nm blue shift of its trans absorption band relative to 2 (FIG. 19b ). On the other hand, its cis isomer's π-π* band is red-shifted by 15 nm. The combined effect is the generation of a pronounced overlap between the absorption bands of the two configurations. This overlap resulted in a drastic decrease in the efficiency of the switch, including its PSS ratio (PSS490=56% and PSS640=79%) and photoswitching quantum yields (Φ_(cis→trans)=51% and Φ_(trans→cis)=42%). The steric hindrance of the ortho-OMe group may destabilize the trans isomer, and prevent the phenyl ring from lying co-planar with the rest of the molecule. The loss of planarity in the trans isomer can be inferred by its blue-shifted absorption band, and its destabilization is evident from the lower trans isomer ratio (58%) under dark.

The BF₂-Azo complexes (4-7) were synthesized with the intention of further pushing the activation wavelength of the systems to even lower energy levels. The attachment of a para-dimethylamino group shifted the absorption peak of complex 4 to 680 nm (£=36,255 M⁻¹ cm⁻¹) with a tail extending out to 760 nm (FIG. 20). This property allowed the trans to cis isomerization process to be activated using NIR light. The UV/Vis spectrum of the trans isomer of 4 exhibits better-resolved vibrational fine structure compared to 2. The half-life of the cis isomer of 4 was determined to be 250 seconds (FIG. 49), and no obvious difference in rate was observed by deoxygenating the methylene chloride solution. The effective bond order of N═N bond is strongly influenced by the substituents on the phenyl rings (vide infra). Electron-donating groups would cause an increase in the electron density of the π* (antibonding) orbital and thus a decrease in the effective bond order of the N═N bond, which leads to a lower thermal isomerization barrier.^(6,20) Under dark, complex 4 is almost quantitatively (97%) composed of the thermodynamically more stable trans form. Upon irradiation with 710 nm light, isomerization to the cis isomer occurs quickly. Because of the fast cis to trans isomerization rate we were unable to record the PSS at 710 nm. The lowest estimation of the amount of cis isomer at PSS710 is 63% based on ¹H NMR spectroscopy (FIG. 35).²¹

The isomerization and photochromic properties of the piperidinyl (5), methylpiperizinyl (6), and morpholinyl (7) BF₂-azo derivatives were studied (FIG. 21). As can be seen by their UV/Vis spectra, these switches all undergo isomerization upon irradiation with 710 nm light, and maintain a high ratio of trans isomer under dark (98%, 94% and 97% trans ratio, with molar extinction coefficient ε=30,458, 29,536 and 28,025 M⁻¹ cm⁻¹, for 5, 6 and 7, respectively). The small variation in the electronic characteristics of the amino groups greatly impacts their photophysical properties, including their absorption bands and half-lives. The overlay of the absorption bands of compounds 5-7 (FIG. 21d ) reveals that the resolution of their vibrational fine structure decreases in the order of 5>6>7, which is in accordance to their para-substituene s electron-donating ability (piperidinyl>methylpiperizinyl>morpholinyl).²² A similar trend is also observed in their absorption maxima (λ_(max)=681 nm, 661 nm and 652 nm for 5, 6 and 7, respectively). In addition, the half-life relaxation rate from cis to trans is substantially enhanced in switch 7 (t_(1/2)=900 s) as compared to 5 (t_(1/2)=150 s) and 6 (t_(1/2)=400 s), which is again in accordance to their electron donation capability. Such a dramatic change in the half-life of these switches was also reflected by their PSS710 values. The highest PSS710 ratio for 5 that could be measured was 23% while for 6 and 7 it was 49% and 83%, respectively. The latter value is comparable to that obtained for the parent azo-BF₂ complex 1.

The stability of the switches in an aqueous environment was tested by conducting multiple switching cycles (10 cycles shown in FIG. 22a ) of 4 in an acetonitrile:PBS buffer (1:1) mixed solvent system. These experiments were conducted by irradiating the sample using 710 nm light, followed by thermal relaxation under dark. Although switch 4 is stable for short periods of time under these conditions, it gradually undergoes hydrolysis (FIG. 50), reverting back to the starting hydrazone²³ compound (FIG. 22b ) with a half-life of 2.3 h. The stability of switch 4 to reduction by glutathione (GSH) was also tested. The switch was incubated in 10 mM reduced glutathione in acetonitrile:PBS buffer (1:1) solution. No obvious difference was observed in its degradation half-life (t_(1/2)=2.5 h) compared to that measured in the acetonitrile/PBS buffer (FIG. 51). The multiple isosbestic points observed in both cases (λ=286 nm, 413 nm and 537 nm) suggest the existence of only two species in solution, which in this case are the azo-BF₂ complex and the hydrazone starting material. These results, along with similar stability tests conducted for compounds 5-7 (FIGS. 52-57), demonstrate the robustness of this class of NIR switches in high concentrations of GSH. Moreover, the results indicate that coordination with BF₂ might be a viable strategy to preventing glutathione reduction of azo compounds.

By comparing the degradation half-life of the complexes in PBS buffer (4≈5>6>7), it is possible to conclude that the higher electron-donating ability of the para-substituent, the more stable the switch is to hydrolysis. According to the crystallographic analysis (FIG. 23), the stronger the electron-donating capability of the para-substituent the longer the N(1)=N(2) bond (1.294(1) and 1.298(2) Å in 2 and 4, respectively) and shorter the B(1)-N(3) bond and B(1)-N(2) bond lengths become (1.550(2) and 1.624(2) Å in 2, and 1.542(2) and 1.615(2) Å in 4, respectively). The latter trend (i.e., the strengthening of the BN bonds) may make the compounds less susceptible to hydrolysis.

In conclusion, this example shows how the para-substitution of azo-BF₂ compounds with electron donating groups leads to photochromic compounds that can be activated with NIR light. Structure property analysis showed that the hydrolysis process in these systems can be modulated and slowed down using strong electron donating para-substituents. Moreover, these switches are not susceptible to reduction by glutathione, most probably because of the coordination with BF₂. These results open the way for using these BF₂-coordinated azo compounds in photopharmacolgical⁴ and opto(chemo)genetical⁵ applications.

REFERENCES

-   (1) Kalka, K.; Merk, H.; Mukhtar, H. J. Am. Acad. Dermatol. 2000,     42, 389-413. -   (2) Smith, K. C., Ed. The Science of Photobiology; Plenum Press: New     York, 1977. -   (3) (a) Irie, M. Chem. Rev. 2000, 100, 1683-1684. (b) Dun, H.;     Bouas-Laurent, H., Eds. Photochromism: Molecules and Systems;     Elsevier: Amsterdam, 2003. -   (4) Velema, W. A.; Szymanski, W.; Feringa, B. L. J. Am. Chem. Soc.     2014, 136, 2178-2191. -   (5) Sjulson, L.; Miesenbcck, G. Chem. Rev. 2008, 108, 1588-1602. (b)     Fehrentz, T.; Schonberger, M.; Trauner, D. Angew. Chem. Int. Ed.     2011, 50, 12156-12182. (b) Brieke, C.; Rohrbach, F.; Gottschalk, A.;     Mayer, G.; Heckel, A. Angew. Chem. Int. Ed. 2012, 51, 8446-8476. -   (6) Bandara, H. M. D.; Burdette, S. C. Chem. Soc. Rev. 2012, 41,     1809-1825. -   (7) (a) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem. Int. Ed.     2007, 46, 72-191. (b) Balzani, V.; Credi, A.; Venturi, M. Molecular     Devices and Machines—Concepts and Perspectives for the Nanoworld;     Wiley-VCH: Weinheim, 2008. (b) Stoddart, J. F. Chem. Soc. Rev. 2009,     38, 1802-1820. (d) Feringa, B. L.; Browne, W. R., Eds. Molecular     Switches; Wiley-VCH: Weinheim, 2011. -   (8) (a) Brash, D.; Rudolph, J.; Simon, J.; Lin, A.; Mckenna, G.;     Baden, H.; Halperin, A.; Ponten, J. Proc. Natl. Acad. Sci. U.S.A.     1991, 88, 1012-10128. (b) Tamai, T. K.; Vardhanabhuti, V.;     Foulkes, N. S.; Whitmore, D. Curr Biol. 2004, 14, R104-R105. (c)     Banerjee, G.; Gupta, N.; Kapoor, A.; Raman, G. Cancer Lett. 2005,     223, 275-284. -   (9) (a) Sadovski, O.; Beharry, A. A.; Zhang, F.; Woolley, G. A.     Angew. Chem. Int. Ed. 2009, 48, 1484-1486. (b) Wegner, H. A. Angew.     Chem. Int. Ed. 2012, 51, 4787-4788. -   (10) (a) Beharry, A. A.; Sadovski, O.; Woolley, G. A. J. Am. Chem.     Soc. 2011, 133, 19684-19687. (b) Bleger, D.; Schwarz, J.; Brouwer,     A.; Hecht, S. J. Am. Chem. Soc. 2012, 134, 20597-20600. (c) Samanta,     S.; Beharry, A. A.; Sadovski, O.; McCormick, T. M.; Babalhavaeji,     A.; Tropepe, V.; Woolley, G. A. J. Am. Chem. Soc. 2013, 135,     9777-9784. (d) Samanta, S.; McCormick, T. M.; Schmidt, S. K.;     Seferos, D. S.; Woolley, G. A. Chem. Comm. 2013, 49, 10314-10316. -   (11) (a) Kurihara, M.; Hirooka, A.; Kume, S.; Sugimoto, M.;     Nishihara, H. J. Am. Chem. Soc. 2002, 124, 8800-8801. (b)     Venkataramani, S.; Jana, U.; Dammaschk, M.; Sönnichsen, F. D.;     Tuczek, F.; Herges. R. Science 2011, 331, 445-448. -   (12) Siewertsen, R.; Neumann, H.; Buchheim-Stehn, B.; Herges, R.;     Nather, C.; Renth, F.; Temps, F. J. Am. Chem. Soc. 2009, 131,     15594-15595. -   (13) Light at 630 nm penetrates only the first 5 mm of tissue, while     light between 700-800 nm has a penetration of 1-2 cm. See ref. 2. -   (14) Wang, L.; Dong, H.; Li, Y.; Xue, C.; Sun, L.-D.; Yan, C.-H.;     Li, Q. J. Am. Chem. Soc. 2014, 136, 4480-4483. -   (15) Sun, L.-D.; Wang, Y.-F.; Yan, C.-H. Acc. Chem. Res. 2014, 47,     1001-1009. -   (16) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am. Chem. Soc. 2012,     134, 15221-15224. -   (17) The para- and/or ortho-substituted derivatives were synthesized     following the synthetic protocol used in the making of the parent     complex 1. See ref. 16. -   (18) Mustroph, H. Dyes Pigm. 1991, 15, 129-137. -   (19) The nature of the effect of oxygen on the isomerization rate is     under investigation. -   (20) Blevins, A. A.; Blanchard, G. J. J. Phys. Chem. B 2004, 108,     4962-4968. -   (21) We are unable to measure the quantum yield of the process     because of the competition with the fast thermal isomerization     process, and instrumentation limitations. -   (22) Mustroph, H. Dyes Pigm. 1991, 16, 223-230. -   (23) (a) Ray, D.; Foy, J. T.; Hughes, R. P.; Aprahamian, I. Nature     Chem. 2012, 4, 757-762. (b) Su, X.; Voskian, S.; Hughes, R. P.;     Aprahamian, I. Angew. Chem. Int. Ed. 2013, 52, 10734-10739. (c)     Tatum, L.; Su, X.; Aprahamian, I. Acc. Chem. Res. 2014, 47,     2141-2149. (d) Su, X.; Aprahamian, I. Chem. Soc. Rev. 2014, 43,     1963-1981.     Supporting Information     General Methods

All reagents and starting materials were purchased from commercial vendors and used without further purification. Column chromatography was performed on silica gel (Silicycle, 230-400 mesh). The melting points were measured on an Electrothermal 9100 instrument in open capillary tubes without thermometer correction. Deuterated solvents were purchased from Cambridge Isotope Laboratories and used as received. ¹H NMR and ¹³C NMR spectra were recorded on a 500 MHz spectrometer, with working frequencies of 499.87 MHz for ¹H nuclei and 125.7 MHz for ¹³C nuclei, respectively. ¹⁹F NMR spectra were recorded on a 300 MHz spectrometer, with working frequency of 282.2 MHz for ¹⁹F nuclei. Chemical shifts are expressed in ppm relative to tetramethylsilane, using the residual solvent peak as a reference standard. GC-MS spectra were measured on a Shimadzu Gas Chromatograph/Mass Spectrometer (GCMS-QP2010Plus). UV-Vis spectra were recorded on a Shimadzu UV-Vis spectrophotometer (UV-1800). Irradiation experiments were conducted with sufficient stirring using a Newport mercury lamp (67005-414) equipped with a Jarrell-Ash light monochromator. The 710 nm Near IR light source was built with appropriate LED light bulbs (LED710-lau AlGaAs Infrared LEDs purchased from Roithner Lasertechnik GmbH). PBS buffer is an aqueous solution prepared with 11.9 mM sodium phosphate (pH 7.3-7.4), 137 mM NaCl and 2.7 mM KCl.

Synthesis

Hydrazone 2:

The compound was synthesized following the procedure described previously.^(S1) 1.7 mL of HBF₄ (48%) was added dropwise to a mixture of p-anisidine (0.420 g, 3.4 mmol) in 1 mL water. After stirring in an ice-bath for 30 min, a precooled aqueous solution of NaNO₂ (1 equiv, 0.235 g, 3.4 mmol) was added dropwise over a period of 15 min. The pinkish diazonium salt was collected by filtration after 60 min and added to a suspension of 2-quinolylacetonitrile^(S2) (0.8 equiv, 0.456 g, 2.7 mmol) and sodium acetate (3.2 equiv, 0.890 g, 10.8 mmol) in a cooled and well stirred 30 mL ethanol/water (2:1) mixture. The resulting reaction mixture was left to stir overnight at RT. The precipitated compound was then collected by filtration and dried over air. The crude product was purified by silica gel column chromatography (hexane/ethyl acetate 9:1) to give Hydrazone 2 as a dark yellow powder (0.572 g, 70%). mp 162.3-162.6° C.; ¹H NMR (500 MHz, CDCl₃) δ 16.18 (s, 1H), 8.29 (d, J=10.0 Hz, 1H), 8.02 (d, J=10.0 Hz, 1H), 7.80 (m, 3H), 7.61 (t, J=7.5 Hz, 1H), 7.42 (d, J=5.0 Hz, 2H), 6.97 (d, J=5.0 Hz, 2H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 157.18, 152.55, 145.76, 137.79, 136.26, 130.92, 128.15, 128.07, 127.62, 126.89, 119.68, 118.43, 116.77, 115.10, 110.34, 55.86 ppm; GC-MS: calcd for C₁₈H₁₄N₄O, 302.1; m/z (rel. inten.) 302 (56%, M⁺), 195 (23%), 128 (34%), 107 (16%), 31 (12%). FIG. 24 shows a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 2 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

2:

The compound was synthesized following the procedure described previously.^(S1) Compound 2 was collected as a purple powder (54% yield). mp 179.8-180.4° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ8.21 (d, J=10.0 Hz, 1H), 7.81 (d, J=10.0 Hz, 2H), 7.76 (t, J=5.0 Hz, 1H), 7.72 (d, J=10.0 Hz, 2H), 7.49 (t, J=7.5 Hz, 1H), 7.34 (d, J=10.0 Hz, 1H), 7.03 (d, J=5.0 Hz, 2H), 3.91 (s, 3H) Ppm; ¹³C NMR (126 MHz, CD₂Cl₂) 6161.30, 152.71, 144.76, 143.49, 139.17, 133.72, 129.60, 125.71, 125.04, 123.28, 118.34, 114.59, 114.20, 113.78, 112.91, 55.88 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ−145.95 (q, J=19.7 Hz, 2F) ppm; GC-MS: calcd for C₁₈H₁₃BF₂N₄O, 350.1; m/z (rel. inten.) 350 (23%, M⁺), 144 (10%), 140 (28%), 107 (100%), 91 (22%), 51 (32%). FIG. 25 shows a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 2-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K. FIG. 26 shows a ¹H NMR spectrum of 2 after being stored in the dark. The equilibrated mixture of 2 was determined to have an isomer ratio of 93:7 (trans:cis). FIG. 27 shows a ¹H NMR spectra of the PSS of 2 at a) 490 nm and b) 630 nm in CD₂Cl₂ at 294 K. The PSS isomer ratios of 92±1% trans at l_(irr)=490 nm, and 96±1% cis at l_(irr)=630 nm are the averages of three experiments.

Hydrazone 3:

The compound was synthesized following the procedure used for Hydrazone 2. Hydrazone 3 was collected as a dark yellow powder (68% yield). mp 164.2-164.7° C.; ¹H NMR (500 MHz, CDCl₃) δ 16.08 (s, 1H), 8.18 (d, J=10.0 Hz, 1H), 7.96 (d, J=5.0 Hz, 1H), 7.75 (m, 3H), 7.65 (d, J=10.0 Hz, 1H), 7.55 (t, J=5.0 Hz, 1H), 6.55 (m, 2H) 4.06 (s, 3H), 3.83 (s, 3H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 157.66, 152.16, 148.93, 145.86, 137.29, 130.68, 128.18, 127.90, 127.34, 126.71, 126.08, 119.39, 118.78, 115.24, 110.68, 105.46, 99.17, 56.31, 55.88 ppm; GC-MS: calcd for C₁₉H₁₆N₄O₂ 332.1; ink (rel. inten.) 332 (66%, M⁺), 137 (21%), 128 (15%), 77 (100%), 51 (13%). FIG. 28 shows a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 3 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

3:

The compound was synthesized following the procedure described previously.^(S1) Compound 3 was collected as a purple powder (48% yield). mp 169.3-169.8° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ8.36 (d, J=10.0 Hz, 1H), 8.06 (d, J=10.0 Hz, 1H), 7.88 (m, 2H), 7.60 (t, J=7.5 Hz, 1H), 7.41 (d, J=10.0 Hz, 1H), 7.21 (d, J=10.0 Hz, 1H), 6.66 (d, J=10.0 Hz, 1H), 6.56 (dd, J=5.0 Hz, 1H), 3.95 (s, 3H), 3.90 (s, 3H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 162.75, 161.85, 144.56, 143.48, 134.16, 133.67, 129.53, 125.73, 121.97, 119.26, 118.38, 114.17, 114.08, 105.14, 104.72, 99.24, 98.03, 56.46, 55.87 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ −150.71 (q, J=25.4 Hz, 2F) ppm; GC-MS: calcd for C₁₉H₁₅BF₂N₄O₂, 380.1; ink (rel. inten.) 380 (24%, M⁺), 207 (70%), 137 (26%), 128 (30%), 73 (100%). FIG. 29 shows a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 3-cis (contains a small percentage of the trans isomer) in CD₂Cl₂ at 294 K. FIG. 30 shows a ¹H NMR spectrum of 3 after being stored in the dark. The equilibrated mixture of 3 was determined to have an isomer ratio of 58:42 (trans:cis). FIG. 31 shows a ¹H NMR spectra of the PSS of 3 at a) 490 nm and b) 640 nm in CD₂Cl₂ at 294 K. The PSS isomer ratios of 56±1% trans at l_(irr)=490 nm, and 79±1% cis at l_(irr)=640 nm are the averages of three experiments.

Hydrazone 4:

The compound was synthesized following the procedure used for Hydrazone 2. Hydrazone 4 was collected as a brown powder (64% yield). mp 170.3-170.7° C.; ¹H NMR (500 MHz, CDCl₃) δ 16.18 (s, 1H), 8.19 (d, J=10.0 Hz, 1H), 7.94 (d, J=10.0 Hz, 1H), 7.75 (m, 3H), 7.55 (t, J=10.0 Hz, 1H), 7.34 (d, J=10.0 Hz, 2H), 6.76 (d, J=5.0 Hz, 2H), 2.98 (s, 3H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 152.73, 148.49, 145.71, 137.41, 137.38, 132.99, 130.70, 127.93, 127.18, 126.65, 119.47, 118.94, 116.83, 113.52, 109.04, 41.09 ppm; GC-MS: calcd for C₁₉H₁₇N₅, 315.2; ink (rel. inten.) 315 (60%, M⁺), 301 (14%), 134 (100%), 119 (46%), 93 (27%), 65 (17%). FIG. 32 shows a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 4 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

4:

The compound was synthesized following the procedure described previously.^(S1) Compound 4 was collected as a green powder (65% yield). mp 193.4-193.8° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ 7.94 (d, J=10.0 Hz, 1H), 7.86 (d, J=10.0 Hz, 2H), 7.72 (d, J=10.0 Hz, 1H), 7.66 (m, 2H), 7.36 (t, J=10.0 Hz, 1H), 7.15 (d, J=10.0 Hz, 1H), 6.80 (d, J=10.0 Hz, 2H), 3.17 (s, 6H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.49, 141.25, 139.42, 132.86, 129.29, 128.25, 128.20, 128.15, 124.48. 124.36, 117.66, 117.64, 114.48, 114.01, 112.11, 40.30 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ −150.45 (q, J=32.0 Hz, 2F) ppm; GC-MS: calcd for C₁₉H₁₆BF₂N₅, 363.1; m/z (rel. inten.) 363 (20%, M⁺), 281 (4%), 207 (11%), 134 (100%), 120 (49%), 65 (17%). FIG. 33 shows a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 4-trans in CD₂Cl₂ at 294 K. FIG. 34 shows a ¹H NMR spectrum of 4 after being stored in the dark. The equilibrated mixture of 4 was determined to have an isomer ratio of 97:3 (trans:cis). FIG. 35 shows a ¹H NMR spectrum recording the lowest estimation of PSS of 4 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 63±1% cis at l_(irr)=710 nm is the average of three experiments.

Hydrazone 5:

The compound was synthesized following the procedure used for Hydrazone 2. Hydrazone 5 was collected as a brown powder (51% yield). mp 168.5-168.9° C.; ¹H NMR (500 MHz, CDCl₃) δ 16.16 (s, 1H), 8.23 (d, J=10.0 Hz, 1H), 7.98 (d, J=10.0 Hz, 1H), 7.78 (m, 3H), 7.58 (t, J=7.5 Hz, 1H), 7.36 (d, J=5.0 Hz, 2H), 6.99 (d, J=10.0 Hz, 2H), 3.17 (t, J=5.0 Hz, 4H), 1.74 (m, 4H), 1.61 (m, 2H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 152.64, 149.95, 145.75, 137.56, 134.91, 130.80, 128.06, 128.00, 127.41, 126.76, 119.59, 118.69, 117.63, 116.52, 109.69, 51.15, 24.45, 22.93 ppm; GC-MS: calcd for C₂₂H₂₁N₅, 355.2; m/z (rel. inten.) 355 (51%, M⁺), 160 (20%), 140 (28%), 84 (17%), 77 (100%), 51 (16%). FIG. 36 shows a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 5 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

5:

The compound was synthesized following the procedure described previously.^(S1) Compound 5 was collected as a green powder (56% yield). mp 187.5-188.0° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ 7.96 (d, J=10.0 Hz, 1H), 7.82 (d, J=7.5 Hz, 2H), 7.73 (d, J=10.0 Hz, 1H), 7.67 (m, 2H), 7.37 (t, J=10.0 Hz, 1H), 7.17 (d, J=10.0 Hz, 1H), 6.95 (d, J=10.0 Hz, 2H), 3.51 (t, J=5.0 Hz, 4H), 1.72 (s, 6H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.69, 141.44, 138.81, 132.94, 129.32, 128.12, 128.04, 124.60, 124.43. 117.72, 117.31, 116.41, 114.47, 113.93, 113.74, 48.59, 25.78, 24.54 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ −150.11 (q, J=18.8 Hz, 2F) ppm; GC-MS: calcd for C₂₂H₂₀BF₂N₅, 403.2; ink (rel. inten.) 403 (14%, M⁺), 355 (10%), 281 (35%), 253 (12%), 207 (87%), 135 (31%), 73 (100%). FIG. 37 shows a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 5-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K. FIG. 38 shows a ¹H NMR spectrum of 5 after being stored in the dark. The equilibrated mixture of 5 was determined to have an isomer ratio of 98:2 (trans:cis). FIG. 39 shows a ¹H NMR spectrum recording the lowest estimation of PSS of 5 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 28±1% cis at l_(irr)=710 nm is the average of three experiments.

Hydrazone 6:

The compound was synthesized following the procedure used for Hydrazone 2. Hydrazone 6 was collected as a brown powder (55% yield). mp 163.5-164.0° C.; ¹H NMR (500 MHz, CDCl₃) δ 16.03 (s, 1H), 8.13 (d, J=10.0 Hz, 1H), 7.87 (d, J=10.0 Hz, 1H), 7.66 (m, 3H), 7.49 (t, J=5.0 Hz, 1H), 7.26 (d, J=10.0 Hz, 2H), 6.90 (d, J=5.0 Hz, 2H), 3.17 (t, J=5.0 Hz, 4H), 2.55 (t, J=5.0 Hz, 4H), 2.32 (s, 3H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 152.21, 147.86, 145.45, 140.06, 137.49, 136.94, 135.74, 130.71, 129.75, 127.64, 127.38, 119.34, 117.60, 116.57, 116.32, 54.39, 48.51, 45.08 ppm; GC-MS: calcd for C₂₂H₂₂N₆, 370.2; ink (rel. inten.) 370 (63%, M⁺), 175 (22%), 140 (36%), 105 (14%), 77 (100%). FIG. 44 shows a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 6 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

6:

The compound was synthesized following the procedure described previously.^(S1) Compound 6 was collected as a green powder (42% yield). mp 181.7-182.1° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ 8.02 (d, J=10.0 Hz, 1H), 7.81 (d, J=5.0 Hz, 2H), 7.70 (m, 3H), 7.40 (t, J=10.0 Hz, 1H), 7.21 (d, J=10.0 Hz, 1H), 6.97 (d, J=7.5 Hz, 2H), 3.48 (t, J=5.0 Hz, 4H), 2.55 (t, J=5.0 Hz, 4H), 2.33 (s, 3H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.46, 152.23, 141.96, 139.70, 139.35, 133.13, 129.39, 127.49, 127.39, 124.89, 124.58, 117.87, 114.40, 113.97, 113.70, 54.85, 47.22, 46.04 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ −149.31 (q, J=18.8 Hz, 2F) ppm; GC-MS: calcd for C₂₂H₂₁BF₂N₆, 418.2; ink (rel. inten.) 418 (20%, M⁺), 355 (12%), 281 (30%), 253 (13%), 207 (84%), 77 (100%). FIG. 45 shows a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 6-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K. FIG. 46 shows a ¹H NMR spectrum of 6 after being stored in the dark. The equilibrated mixture of 7 was determined to have an isomer ratio of 97:3 (trans:cis). FIG. 47 shows a ¹H NMR spectrum recording the lowest estimation of PSS of 6 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 49±1% cis at l_(irr)=710 nm is the average of three experiments.

Hydrazone 7:

The compound was synthesized following the procedure used for Hydrazone 2. Hydrazone 7 was collected as a brown oil (56% yield). ¹H NMR (500 MHz, CDCl₃) δ 16.08 (s, 1H), 8.20 (d, J=10.0 Hz, 1H), 7.94 (d, J=10.0 Hz, 1H), 7.75 (m, 3H), 7.56 (t, J=7.5 Hz, 1H), 7.34 (d, J=10.0 Hz, 2H), 6.95 (d, J=10.0 Hz, 2H), 3.88 (t, J=5.0 Hz, 4H), 3.17 (t, J=5.0 Hz, 4H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 152.47, 148.83, 145.67, 137.60, 135.63, 130.86, 128.06, 128.00, 127.52, 126.76, 119.54, 118.52, 116.91, 116.54, 110.04, 69.09, 49.84 ppm; GC-MS: calcd for C₂₁H₁₉N₄O, 357.2; m/z (rel. inten.) 357 (64%, M⁺), 162 (18%), 140 (31%), 105 (18%), 77 (100%). FIG. 40 shows a) ¹H NMR and b) ¹³C NMR spectra of Hydrazone 7 (contains a small percentage of the Z isomer) in CDCl₃ at 294 K.

7:

The compound was synthesized following the procedure described previously.^(S1) Compound 7 was collected as a green powder (45% yield). mp 174.8-175.3° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ 8.06 (d, J=5.0 Hz, 1H), 7.81 (d, J=7.5 Hz, 2H), 7.73 (m, 3H), 7.42 (t, J=7.5 Hz, 1H), 7.24 (d, J=10.0 Hz, 1H), 6.97 (d, J=10.0 Hz, 2H), 3.86 (t, J=5.0 Hz, 4H), 3.41 (t, J=5.0 Hz, 4H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 152.44, 142.31, 140.35, 139.30, 133.32, 129.52, 129.39, 127.09, 126.92. 125.09, 124.69, 117.97, 114.36, 114.07, 113.52, 66.65, 47.51 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ −148.73 (q, J=18.8 Hz, 2F) ppm; GC-MS: calcd for C₂₁H₁₈BF₂N₅O, 405.2; m/z (rel. inten.) 405 (26%, M⁺), 355 (10%), 281 (32%), 253 (11%), 207 (83%), 135 (37%), 73 (100%). FIG. 41 shows a) ¹H NMR b) ¹³C NMR and c) ¹⁹F NMR spectra of 7-trans (contains a small percentage of the cis isomer) in CD₂Cl₂ at 294 K. FIG. 42 shows a ¹H NMR spectrum of 7 after being stored in the dark. The equilibrated mixture of 7 was determined to have an isomer ratio of 94:6 (trans:cis). FIG. 43 shows a ¹H NMR spectrum recording the lowest estimation of PSS of 7 at 710 nm in CD₂Cl₂ at 294 K. The PSS isomer ratio of 82±1% cis at l_(irr)=710 nm is the average of three experiments.

8:

The compound was synthesized following the procedure described previously.^(S1 1)H NMR (500 MHz CD₂Cl₂) δ: 7.81, 7.76, 7.62, 7.55, 7.47, 7.24, 7.03, 6.68, 4.33, 3.56, 1.75, 1.47

9:

Compound 9 includes a ester linked cholesterol substituent in the para position. The compound starts to decompose at 195.5° C. before reaching its mp; ¹H NMR (500 MHz, CD₂Cl₂) δ=8.41 (d, J=8.9 Hz, 1H), 8.13 (dd, J=6.8, 1.9 Hz, 2H), 7.91 (d, J=7.9 Hz, 1H), 7.84 (m, 2H), 7.59 (t, J=8 Hz, 1H), 7.53 (d, J=8.6 Hz, 2H), 7.49 (d, J=8.9 Hz, 1H), 5.37 (s, 1H), 4.86 (m, 1H), 2.50 (m, 1 h), and 2.11-0.062 (cholesterol skeleton). ¹³C NMR (126 MHz, CD₂Cl₂) δ=165.22, 152.68, 144.96, 140.02, 139.11, 134.41, 131.12, 130.25, 130.22, 129.77, 128.95, 126.77, 125.73, 122.88, 122.40, 118.87, 114.22, 75.00, 71.89, 65.69, 56.97, 56.39, 54.08, 53.86, 53.65, 53.43, 53.21, 50.35, 42.52, 40.01, 39.71, 38.40, 37.26, 36.87, 36.40, 36.04, 32.17, 32.11, 30.78, 28.42, 28.24, 28.07, 27.96, 24.47, 24.02, 22.77, 22.52, 21.27, 19.40, 19.37, 19.10, 18.72, 13.71, 11.84 ppm; ¹⁹F NMR (282 MHz, CD₂Cl₂) δ=−150.66 (m, 2F); GC-MS: calcd. for C₄₅H₅₅BF₂N₄O₂732.33; m/z (rel. inten.) 732.2 (7.5%, M+).

Kinetics Studies

A 0.1 mM deoxygenated CH₂Cl₂ solution of compound 2 in a quartz cuvette was irradiated at 630 nm until its photostationary state was reached (no further changes in the UV-Vis spectra were observed). The thermal isomerization process was monitored by measuring the change in absorption intensity at 594 nm as a function of time (at 1 min intervals). The half-life (t_(1/2)) of the cis→trans isomerization for 2 was calculated to be 10.4 h, which is the average of three measurements conducted under the same conditions. When the rate was measured in an unaltered solution, the half-life of 2 went down to 25 min. The half-life of 3 was measured as well in deoxygenated CH₂Cl₂, and was determined to be 13.4 h.

Unaltered CD₂Cl₂ solution of compound 4 in a quartz NMR tube was irradiated at 710 nm until its photostationary state was reached. The thermal isomerization process was monitored by measuring the change in the integrations of diagnostic peaks of both cis and trans isomers as a function of time (at 30 seconds intervals). The half-life (t_(1/2)) of the cis→trans isomerization was calculated to be 250 s, which is the average of three measurements conducted under the same conditions. No obvious change was observed in deoxygenated samples of 4. Similar kinetics experiments were carried out for compounds 5-7, and their half-lives are summarized in Table B.

TABLE B The cis→trans thermal relaxation rates for 4-7 in CD₂Cl₂ at 294 K. Compound 4 5 6 7 Half-life 250 s 150 s 400 s 900 s X-Ray Crystallography

Data were collected using a Bruker CCD (charge coupled device) based diffractometer equipped with an Oxford Cryostream low-temperature apparatus operating at 173 K. Data were measured using omega and phi scans of 1° per frame for 30 s. The total number of images was based on results from the program COSMO^(S3) where redundancy was expected to be 4.0 and completeness to 0.83 Å to 100%. Cell parameters were retrieved using APEX II software^(S4) and refined using SAINT on all observed reflections. Data reduction was performed using the SAINT software^(S5) which corrects for Lp. Scaling and absorption corrections were applied using SADABS^(S6) multi-scan technique, supplied by George Sheldrick. The structures are solved by the direct method using the SHELXS-97 and refined by least squares method on F², SHELXL-97, which are incorporated in SHELXTL-PC V 6.10.^(S7) The structure of 2 and 4 were solved in the space group P2₁/c. All non-hydrogen atoms are refined anisotropically. Hydrogens were calculated by geometrical methods and refined as a riding model. Crystals used for the diffraction studies showed no decomposition during data collection.

Preparation of the Crystal Sample of 2:

Compound 2 (20 mg) was dissolved in 3 mL methylene chloride. The solution was stored in the dark and allowed to evaporate over 2 days. Dark yellow plate crystals were collected.

Preparation of the Crystal Sample of 4:

Compound 4 (20 mg) was dissolved in 3 mL methylene chloride. The solution was stored in the dark and allowed to evaporate over 2 days. Red block crystals were collected.

TABLE C Crystal Data and Parameters for 2 and 4. 2 4 Empirical formula C₁₈H₁₃BF₂N₄O C₁₉H₁₆BF₂N₅ Formula weight 350.13 363.18 Temperature 172.99 K 173.15 K Wavelength 1.54178 Å 0.71073 Å Crystal system Monoclinic Monoclinic Space group P 2₁/c P 2₁/c Unit cell dimensions a = 16.9594(2) Å a = 17.6056(13) Å α = 90° α = 90° b = 12.7408(2) Å b = 13.5354(10) Å β = 94.1250(1)° β = 100.3040(10)° c = 7.2440(1) Å c = 7.3372(5) Å γ = 90°. γ = 90°. Volume 1561.20(4) Å³ 1720.2(2) Å³ Z 4 4 Density 1.490 Mg/m³ 1.402 Mg/m³ (calculated) Absorption 0.938 mm⁻¹ 0.101 mm⁻¹ coefficient F(000) 720 752 Crystal size 0.279 × 0.171 × 0.391 × 0.198 × 0.05 mm³ 0.132 mm³ Theta range 8.688 to 136.638° 3.82 to 50.74° for data collection Index ranges −20 <= h <= 19, −15 <= −21 <= h <= 21, −16 <= k <= 15, −8 <= l <= 8 k <= 16, −8 <= l <= 8 Reflections 11595 27436 collected Independent 2863 [R(int) = 3145 [R(int) = reflections 0.0249] 0.0433] Goodness-of-fit 1.037 1.050 on F² Final R indices R₁ = 0.0315, R₁ = 0.0365, [I > 2σ(I)] ωR₂ = 0.0877 ωR₂ = 0.0874 R indices (all data) R₁ = 0.0372, R₁ = 0.0538, ωR₂ = 0.0923 ωR₂ = 0.0954

REFERENCES

-   (S1) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am. Chem. Soc. 2012,     134, 15221-15224. -   (S2)(a) Lipshutz, B. H.; Kim, S.; Mollard, P.; Blomgren, P. A.;     Stevens, K. L. Tetrahedron 1998, 54, 6999-7012. (b) Domasevitch, K.     V.; Gerasimchuk, N. N.; Mokhir, A. Inorg. Chem. 2000, 39, 1227-1237. -   (S3) COSMO V1.61, Software for the CCD Detector Systems for     Determining Data Collection Parameters. Bruker Analytical X-ray     Systems, Madison, Wis. (2009). -   (S4) APEX2 V2010.11-3. Software for the CCD Detector System; Bruker     Analytical X-ray Systems, Madison, Wis. (2010). -   (S4) SAINT V 7.68A Software for the Integration of CCD Detector     System Bruker Analytical X-ray Systems, Madison, Wis. (2010). -   (S5) Blessing, R. H. Acta Cryst. A 1995, 51, 33-38. -   (S6) Sheldrick, G. M. Acta Cryst. A 2008, 64, 112-122.

Example 5: Controlling the Isomerization Rate of an Azo-BF₂ Switch Using Aggregation

A novel visible-light activated azo-BF₂ switch having an extended phenanthridinyl π system has been synthesized and its switching properties characterized as a function of concentration. The switch can self-aggregate into large assemblies through π-π interactions in concentrated solutions and its Z→E thermal isomerization rate can be modulated by the degree of aggregation. This property allowed the inventors to tune the relaxation half-life of the same switch from seconds to days.

The modulation of the prolyl cis-trans isomerization rate by peptidyl prolyl cis-trans isomerases (PPIases) results in a molecular timer that regulates a variety of physiological events, such as cell cycle, cell signaling, and gene expression.¹⁻³ A similar control over the isomerization rate of photochromic compounds can enable the development of photoactive molecular timers that can be used not only in the control of molecular motions and functions but their timing as well. Such a control over spatial and temporal order will ultimately enable the mimicking of the complexity found in nature. While photoswitchable molecules have been extensively studied and applied in areas ranging from biological systems to material science,⁴⁻⁷ the active modulation of the isomerization rates of such systems has not been explored in-depth. Azobenzene is one of most popular photochromic compounds found in the literature, mainly because of its ease of synthesis, tunable wavelength, and high photostability, among other properties.⁸ While highly useful, azobenzenes have some drawbacks that still need to be addressed, such as the use of UV light for its E to Z isomerization. Recently there has been some progress in these areas, for example, Hecht,⁹ Woolley,¹⁰ and Herges¹¹ have developed systems that can be activated with visible light.¹²

In addition to activation wavelength for photoisomerization, the thermal relaxation half-life is another intrinsic property of photochromic compounds that needs to be well tuned. In general, each azobenzene derivative has an invariable half-life at constant temperature. Such half-life values can range from ns to days, where different half-lives may be associated with different functionalities. For example, Velasco and co-workers found the fastest azopyrimidine photoswitch (t_(1/2) down to 40 ns at 298 K), which can be a potential chromophore for real-time information-transmission in organisms.¹³ While Hecht et al designed fluorinated azobenzenes with half-lives of up to 700 days¹⁴ that can potentially be used in the development of ultrahigh-density optical data storage materials.¹⁵ In most cases though, changing the thermal stability of photoswitches relies on molecular structure modification that can be a time-consuming, tedious and low yielding process. Moreover, once the structural variations are made the half-life of the system cannot be changed.

The present inventors found a new methodology to address one of the shortcomings of azobenzenes, i.e., UV light activation. This was achieved by BF₂ coordination leading to azo-BF₂ complexes that can be induced to isomerize using visible,¹⁶ or near-infrared (NIR) light sources.¹⁷ These systems are highly efficient, and show slow to fast Z to E isomerization rates. During structure property analyses the inventors found out that electron donating para-substituents red-shift the activation wavelength of these systems; however, this effect is accompanied with a drastic shortening of the isomerization half-lives, especially of the NIR activated systems. To investigate whether the expansion of the π-system from a quinolinyl to a phenanthridine group would lead to a red-shift in the activation wavelength without the deleterious effect on the isomerization rate, the synthesis of compound 10 was targeted. The designed BF₂-azo switch (10) indeed shows some red-shift in activation wavelength vs. the parent system. More importantly, the system forms supramolecular aggregates, the degree of which drastically modulates the Z→E thermal isomerization rate. This aggregation behavior allowed control of the isomerization rate of the same compound by a factor of 1800 (from seconds to days). This unprecedented finding (specifically in the azobenzene context) opens a new way for controlling the thermal properties of photochromic compounds.

The synthesis of the azo-BF₂ complex 10 started from the hydrazone-based switch,¹⁸ followed by the coupling reaction with BF₃.OEt₂ ¹⁹ (Scheme 7). The product was fully characterized using NMR spectroscopy and mass spectrometry (FIGS. 62-76). An equilibrated solution (under dark) of 10 shows an absorption maximum (λ_(max)) at 535 nm, with an absorption coefficient constant (e) of 12356 M⁻¹·cm⁻¹ (FIG. 79), which is assigned to the E isomer. Upon irradiation with a 600 nm light source, E to Z isomerization takes place (isosbestic points at 318, 406, and 508 nm) leading to a new band with a λ_(max) at 501 nm, and e of 11209 M⁻¹·cm⁻¹ (FIGS. 58 and 81). The isomerization is accompanied with a significant color change of the solution from purple to dark orange (FIG. 58b ). This type of negative photochromism while rare²⁰ is typical to the Azo-BF₂ switches.^(16, 17) As expected the E and Z isomers exhibit 5 and 21 nm bathochromic shifts, respectively, compared to the absorption maxima of the parent azo-BF₂ switch (Scheme 6).¹⁶ These shifts are attributed to the larger π-conjugation of phenanthridinyl ring system. The Z isomer switches back to E upon 430 nm light irradiation (FIG. 58b ) with clear isosbestic points (318, 406, and 508 nm), indicating that only two species participate in the switching cycle. This reversible E/Z isomerization process can be repeated multiple times without fatigue (up to 50 cycles) upon alternative visible light irradiation cycles (FIG. 58c ).

¹H NMR spectroscopy was employed to evaluate the photo-switching efficiency of 10 in CD₂Cl₂ (0.53 mM). The equilibrated switch (under dark) exists as a 97:3 mixture of E:Z isomers (FIG. 77a ). The E-dominant sample was irradiated using 600 nm light for 60 s to reach a photostationary state (PSS) consisting of 91% Z isomer (FIG. 77b ). The quantum yield (Φ) for this process was calculated to be 55±4%. As for the Z→E isomerization process, 430 nm light irradiation for 10 s yields a PSS₄₃₀ consisting of 65% E isomer with Φ_(Z→E) of 77±3% (FIG. S77 c).²¹

Single crystallographic analysis (FIG. 59a ) reveals that the azo-BF₂ complex adopts a E configuration in the solid state. The bond lengths of N(1)=N(2) (1.280(2) Å), B(1)-N(1) (1.638(3) Å), and B(1)-N(3) (1.563(3) Å) are almost identical to those of the parent azo-BF₂ compound.¹⁶ The C(2)-N(3) bond retains a double bond character, (1.341(3) Å) which determines the planarity of phenanthridinyl group in BF₂-azo single crystal.²²

It is well documented²³ that phenanthridinium can self-associate into aggregates through π-π stacking. Single crystallographic analysis of the azo-BF₂ complex (FIG. 59a ) showed that the switch also aggregates into a cylindrical structure through head-to-head π-π stacking (3.4956(0) Å, and 3.4658(0) Å) (FIG. 59b ).²⁴ This inspired the study of whether such an aggregation might occur in solution as well. The self-aggregation behavior of 10 in CH₂Cl₂ was studied using UV-Vis spectroscopy and no aggregation was observed for either the E or Z isomers at a concentration below 1×10⁴ M (FIGS. 78-81).²⁵ In other words, 10 is monomeric under these conditions. Next, self-aggregation of 10 was investigated at a higher concentration range using ¹H NMR spectroscopy (FIG. 82).²⁶

At 29.0 mM (FIG. 82), the proton signals in the ¹H NMR spectrum become broad, owing to a dynamic exchange process between monomeric and multimeric species in solution, indicating the existence of large self-aggregated species.²⁷ The resonance signals gradually become sharp upon dilution, as a result of the disassembly of the aggregates. Consequently, the phenanthridinyl protons H1-8 are slight downfield shifted, while the chemical shifts of the phenyl protons H9-11 remain almost unchanged (FIG. 82). The downfield shift of the proton signals stems from the dissociation of the π-π interactions between the phenanthridinyl rings.²⁸ This result indicates that the azo-BF₂ molecule adopts a similar head-to-head aggregation pattern in concentrated solution, to the one observed in the solid state (FIG. 59b ).²⁴ DOSY (diffusion ordered spectroscopy) NMR spectroscopy experiments at the 8.26, 1.51, 0.53, and 0.15 mM were performed to study the decrease in the size of the aggregates upon diluting the samples. As expected, the diffusion coefficient (D) increased from 1.238×10⁻⁹ to 1.952×10⁻⁹ m²/s while decreasing the sample concentration from 8.26 to 0.15 mM, indicating the disassembly of the aggregates. (FIGS. 84-87).

To answer the question of how this aggregation behavior influences the thermal isomerization property of the azo-BF₂ switch, the Z→E thermal relaxation of 10 was monitored at different concentrations using UV-Vis and NMR spectroscopies. The half-life of a sample at 2.25×10⁻⁵ M, where 10 is in the monomeric form, was determined to be 389 s (FIG. 88). This value is around 4 times smaller than the half-life of the parent BF₂-azo compound.¹⁶ Surprisingly, the half-life of 10 is enhanced 1800 times to about 8.1 days, when the concentration is increased to 8.26 mM (FIGS. 60 and 100-103). Such an unusual concentration-dependent thermal relaxation has not been observed, to the best of the inventors' knowledge, in azobenzenes.³⁰ The analysis of the rate data (k) as a function of concertation showed the existence of a linear relationship between the double logarithmic plot of k as function of concentration (FIG. 98). Based on this correlation it should be possible to dial into a large range of function-dependent isomerization rates, just by changing the concentration of the same molecular switch. This finding opens the door to a completely new strategy for the control of the isomerization rates of photochromic compounds. A possible explanation for this aggregation induced half-life enhancement is the prevalence of the rotation rather than inversion isomerization mechanism in non-substituted azo-BF₂ compounds.³¹ Upon aggregation, rotation will be hindered by neighboring molecules, leading to the observed slowing down of the isomerization rate.³²

The dependency of the isomerization half-life on the aggregation state enabled the inventors to introduce concentration as a second stimulus by which to control the switching of 10. The in-situ temporal control was achieved by consecutively diluting and concentrating a solution of 10 between 1.73 and 1.20 mM and monitoring the rate of the Z→E isomerization process using NMR spectroscopy. At 1.73 mM, the half-life was determined to be 132±2 h; after dilution the half-life decreased to 117±3 h (FIG. 61). This reversible half-life switching was repeated up to two and a half cycles, by which time the volume of the NMR tube precluded continuing dilutions (FIGS. 93-97).

In conclusion, a novel visible light induced azo-BF₂ switch having an extended π system has been synthesized. This switch can self-aggregate into large assemblies in concentrated solutions as well as in the solid state through head-to-head π-π interactions between its phenanthridinyl groups. Uniquely, the thermal Z→E isomerization rate displays a linear dependency on the degree of aggregation in solution; the higher the concentration, the bigger the size of aggregates, and the slower the isomerization rate. This correlation can lead to surfaces, materials and photoactivatable drugs having different response rates depending on coverage and switch concentration.

REFERENCES

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All reagents and starting materials were purchased from commercial vendors and were used as supplied unless otherwise indicated. All experiments were conducted in air unless otherwise noted. Column chromatography was performed on silica gel (SiliCycle®, 60 A, 230-400 mesh). Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. and used as received. ¹H NMR, ¹³C NMR and 2D NMR spectra were recorded on a 500 MHz NMR spectrometer, with working frequencies of 499.87 MHz for ¹H nuclei, and 125.7 MHz for ¹³C nuclei. ¹⁹F NMR spectra were recorded on 600 MHz spectrometer, with working frequency of 564.7 MHz, respectively. Chemical shifts are quoted in ppm relative to tetramethylsilane (TMS), using the residual solvent peak as the reference standard. Hi-Res mass spectra were obtained on a Micromass Q-Tof Ultima ESI mass spectrometer. Melting points were measured on an Electrothermal Thermo Scientific IA9100X1 digital melting point instrument. UV-Vis spectra were recorded on a Shimadzu UV-1800 UV-Vis spectrophotometer.

Irradiation experiments were conducted with 600 (a AlInGaP based LED with radiation of 5 mW, purchased from Roithner Lasertechnik GmbH) and 430 nm (a hyper violet LED with radiation of 880-960 mW at 700 mA, purchased from LEDGroupBuy) LED light bulbs. The LEDs were affixed in a custom irradiation chamber fitted with a cooling fan to maintain ambient temperature. The light intensities at 430 and 600 nm were determined using chemical actinometry^(S1) and a Thorlabs optical power meter, respectively. Photoisomerization quantum yields were measured by monitoring the proton integral change in trans/cis isomers after 430 and 600 nm irradiation for 2 and 20 s, respectively. Each quantum yield measurement was repeated three times under the same conditions. The photostationary states (PSS) were determined upon continuous irradiation until no further isomerization was observed using ¹H NMR spectroscopy. PBS buffer is an aqueous solution prepared by dissolving Na₂HPO₄ (1.420 g), KH₂PO₄ (0.245 g) NaCl (8.007 g) and KCl (0.201 g) in 1 L deionized water. The pH of the resulting buffer solution was adjusted to 7.3-7.4 with 1M sodium hydroxide solution to reach a final concentration of 10 mM PO₄ ³⁻, 137 mM NaCl and 2.7 mM KCl.

Synthesis

16:

This compound was synthesized using a modified reported procedure.^(S2) Chloroacetyl chloride (1.5 mL, 18.9 mmol) was added to a pre-cooled THF (7.5 mL), and the solution was stirred at 0° C. for 10 min. A solution of [1,1′-biphenyl]-2-amine (1.0 equiv, 3.2 g, 18.9 mmol) in 5.5 mL THF was added dropwise over a period of 20 min to the chloroacetyl chloride solution, and the resulting solution was continuously stirred at room temperature (r.t.). After 2 hours, trimethylamine (TEA, 1.0 equiv, 18.9 mmol) was added, and the reaction mixture was stirred for another 2 hours at r.t. 24 mL of water were added and the reaction mixture produced a thick crystalline slurry. The crude product was collected by filtration and purified by silica gel column chromatography using 5:1 hexanes/ethylacetate as eluent to give 16 as a beige powder (4.41 g, 95%). The identity of 16 was confirmed by comparing the obtained ¹H NMR with the published one.^(S2 1)H NMR (500 MHz, CDCl₃) δ 8.48 (s, 1H), 8.38 (d, J=8.3 Hz, 1H), 7.55-7.39 (m, 6H), 7.32 (dd, J=7.6, 1.6 Hz, 1H), 7.25 (td, J=7.5, 1.2 Hz, 1H), 4.10 (s, 2H) ppm.

15:

This compound was synthesized using a modified reported procedure.^(S3)

Trifluoromethanesulfonic anhydride (Tf₂O, 1.5 equiv, 4.17 mL, 23.8 mmol) was added dropwise to a solution of triphenylphosphine oxide (Ph₃PO, 3 equiv, 13.25 g, 47.7 mmol) in anhydrous dichloromethane (CH₂Cl₂, 156 mL) over a period of 10 min under N₂ atmosphere at 0° C. The reaction mixture was continuously stirred for 30 min at 0° C. Amide 15 (3.91 g, 15.9 mmol) in anhydrous CH₂Cl₂ (10 mL) was then added. The reaction was left to warm up, and stirred at this temperature for 5.5 hrs. Saturated bicarbonate was added to quench the reaction, and the mixture was extracted with CH₂Cl₂ (3×100 mL). The combined organic extracts were washed with brine, and finally dried over magnesium sulfate (MgSO₄). The crude product was subjected to silica gel column chromatography using 6:1 hexanes/ethylacetate as eluent to give 15 as a light yellow solid (3.35 g, 92%). The identity of 15 was confirmed by comparing the obtained ¹H NMR with the published one.^(S4 1)H NMR (500 MHz, CDCl₃) δ 8.67 (d, J=8.3 Hz, 1H), 8.57 (d, J=9.2 Hz, 1H), 8.36 (d, J=8.2 Hz, 1H), 8.15 (d, J=8.1 Hz, 1H), 7.88 (t, J=8.2 Hz, 1H), 7.78-7.65 (m, 3H), 5.20 (s, 2H) ppm.

14:

This compound was synthesized using a modified reported procedure.^(S5) Compound 15 (1.5 g, 6.61 mmol) was dissolved in N,N-dimethylformamide (DMF, 58 mL), and the solution was stirred and heated to 90° C., while a solution of sodium cyanide (1.2 equiv, 0.389 g, 7.93 mmol) in 4.5 mL of water was added dropwise over a period of 15 min. The reaction mixture was maintained at 90° C. and stirred overnight. After cooling to r.t., the filtrate was collected by filtration and concentrated under vacuum. The crude product was redissolved in CH₂Cl₂, washed with water and brine, and dried over MgSO₄. The organic phase was concentrated under vacuum and the residue was purified by flash silica gel column chromatography using 3:1 hexanes/ethyl acetate as eluent to give 14 as a yellow solid (0.94 g, 65%). m.p. 116.6-117.0° C.; ¹H NMR (500 MHz, CDCl₃) δ 8.61 (d, J=8.3 Hz, 1H), 8.50 (dd, J=8.1, 1.4 Hz, 1H), 8.11 (dd, J=8.1, 1.1 Hz, 1H), 8.06 (dd, J=8.2, 0.4 Hz, 1H), 7.84 (ddd, J=8.3, 7.1, 1.2 Hz, 1H), 7.70 (t, J=7.7 Hz, 2H), 7.64 (ddd, J=8.3, 7.1, 1.4 Hz, 1H), 4.38 (s, 2H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ 150.09, 143.22, 133.18, 131.24, 130.09, 129.12, 127.99, 127.73, 125.00, 124.12, 124.01, 122.86, 122.04, 116.54, 26.05 ppm; Hi-Res MS (ESI): m/z found [M-H⁺] for C₁₅H₁₁N₂ ⁺ 219.0916 (calcd. 219.0922).

13:

Hydrazone 13 was synthesized following a modified reported procedure.^(S6) 48 wt % HBF₄ (2.5 mL) was added dropwise to aniline (500 mg, 5.36 mmol), followed by the addition of 1.25 mL of water. After stirring at 0° C. for 30 min, a pre-cooled solution of sodium nitrite (NaNO₂, 1.2 equiv, 445 mg, 6.45 mmol) in 1.25 mL water was added dropwise to the reaction mixture over a period of 30 min, followed by 60 min stirring at 0° C. The corresponding diazonium salt precipitate was collected by filtration, and washed with diethyl ether (Et₂O). Sodium acetate (NaOAc, 4 equiv, 377 mg, 4.6 mmol) in 1 mL water was added to a solution of 14 (250 mg, 1.15 mmol) in 5 mL C₂H₅OH/CH₂Cl₂ (4:1). After stirring at r.t. for 60 min, the above suspension was cooled to 0° C., followed by a dropwise addition of the prepared diazonium tetrafluoroborate (1 equiv, 221 mg, 1.15 mmol). The resulting reaction mixture was stirred overnight at r.t. The solution was further diluted with water, extracted with CH₂Cl₂, washed with bicarbonate and brine, and dried over MgSO₄. The solvent was removed under reduced pressure and the residue was subject to silica gel column chromatography using 6:1 hexanes/ethyl acetate as eluent to give 13 as a yellow solid (274 mg, 74%). m.p. 216.0-216.3° C.; ¹H NMR (500 MHz, CD₂Cl₂) δ 9.40 (d, J=8.4 Hz, 1H), 9.20 (s, 1H), 8.74 (d, J=8.2 Hz, 1H), 8.62 (d, J=8.2 Hz, 1H), 8.19 (dd, J=8.1, 1.1 Hz, 1H), 7.94 (t, J=8.3 Hz, 1H), 7.87-7.76 (m, 2H), 7.73 (t, J=8.2 Hz, 1H), 7.52-7.36 (m, 4H), 7.16 (t, J=7.3 Hz, 1H) ppm; ¹³C NMR (151 MHz, CD₂Cl₂) δ 141.75, 131.64, 130.87, 130.00, 129.77, 129.57, 129.47, 129.12, 128.86, 128.36, 128.03, 127.93, 127.88, 127.53, 125.88, 123.81, 123.37, 122.46, 122.09, 115.32, 114.75 ppm; Hi-Res MS (ESI): m/z found [M-H⁺] for C₂₁H₁₅N₄ ⁺ 323.1300 (calcd. 323.1297).

10:

This compound was synthesized following a modified reported procedure.^(S7) Hydrazone 13 (64 mg, 0.2 mmol) was dissolved in 15 mL CH₂Cl₂, and then the solution was transferred into a flame-dried round bottom flask under N₂ gas protection. BF₃.OEt₂ (10 equiv, 0.26 mL, 2.0 mmol) in 3 mL CH₂Cl₂ was added dropwise to a solution of hydrazone in a period of 30 min, during which a bright orange precipitate developed. N,N-Diisopropylethylamine (DIPEA, 7 equiv, 0.24 mL, 1.4 mmol) was added dropwise to the above suspension, and the resulting reaction mixture was stirred under dark for 24 hrs. During the reaction, the orange precipitate disappeared and the color of solution turned into dark red. The crude mixture was concentrated under vacuum, and then subjected to silica gel column chromatography under dark using 6:1 hexanes/ethyl acetate and 1:3 CH₂Cl₂/hexanes as eluents to give 10 as a dark brown crystalline solid (20 mg, 27%). m.p. 239.6-239.9° C.; ¹H NMR (600 MHz, CD₂Cl₂) δ 9.21 (d, J=8.3 Hz, 1H), 8.65 (d, J=8.3 Hz, 1H), 8.51 (d, J=8.3 Hz, 1H), 8.08 (t, J=7.7 Hz, 1H), 7.89 (d, J=8.4 Hz, 1H), 7.86 (t, J=7.7 Hz, 1H), 7.70 (t, J=7.8 Hz, 1H), 7.60 (m, 3H), 7.53 (t, J=7.8 Hz, 2H), 7.43 (t, J=7.4 Hz, 1H) ppm; ¹³C NMR (126 MHz, CD₂Cl₂) δ 135.25, 130.96, 130.89, 129.18, 128.83, 128.71, 128.45, 128.34, 127.80, 126.87, 126.03, 124.65, 123.27, 123.23, 123.14, 122.88, 122.74, 119.44, 119.37 ppm; ¹⁹F NMR (565 MHz, CD₂Cl₂) δ −142.25 (dd, J=62.0, 28.4 Hz) ppm. Hi-Res MS (ESI): m/z found [M-H⁺] for C₂₁H₁₄BF₂N₄ ⁺ 371.1274 (calcd. 371.1279).

12:

This compound was obtained during the same synthesis of compound 10. After purification by multiple times of silica gel column chromatography under dark using 6:1 hexanes/ethyl acetate and 1:3 CH₂Cl₂/hexanes as eluents, 12 was collected as a dark red crystalline powder (24 mg, 32%). m.p. 250.3-250.6° C.; ¹H NMR (600 MHz, CD₂Cl₂) δ 9.45 (d, J=8.6 Hz, 1H), 8.86-8.79 (m, 1H), 8.67 (d, J=8.4 Hz, 1H), 8.60 (dd, J=8.2, 1.4 Hz, 1H), 8.06 (t, J=8.2 Hz, 1H), 7.85 (ddd, J=17.2, 8.5, 4.4 Hz, 3H), 7.80-7.72 (m, 2H), 7.42 (dd, J=8.5, 7.5 Hz, 2H), 7.29 (t, J=7.4 Hz, 1H) ppm; ¹³C NMR (151 MHz, CD₂Cl₂) δ 144.20, 135.36, 134.91, 134.56, 130.70, 129.08, 128.94, 128.87, 128.49, 127.45, 125.06, 124.62, 124.56, 124.49, 122.87, 122.77, 120.98, 120.01, 118.86 ppm; ¹⁹F NMR (565 MHz, CD₂Cl₂) δ −124.34 (dd, J=70.0, 34.9 Hz) ppm. Hi-Res MS (ESI): m/z found [M-H⁺] for C₂₁H₁₄BF₂N₄ ⁺ 371.1274 (calcd. 371.1279).

Concentration-Dependent UV-Vis Studies

Concentration-dependent UV-Vis spectroscopy studies of compound 10 were performed to evaluate its aggregation processes at low concentrations. At concentration below 1.19×10⁻⁴ M, the molecule does not show any aggregation phenomena neither in the trans- or cis-configuration. This is evident by the fact that the absorbance change at λ_(max)=535 or 501 nm (for trans and cis, respectively) as a function of concentration follows the Lambert-Beer law.^(S8) Once the concentration is increased higher than 1.19×10⁻⁴ M, the absorption peak becomes too strong to be observed.

Concentration-Dependent NMR Studies

Next ¹H NMR spectroscopy was employed to investigate the aggregation of 10 at a higher concentration range. The solutions of BF₂-azo 10 in CD₂Cl₂ with estimated concentrations were prepared by diluting a stock solution. The accurate concentrations were calculated and confirmed through UV-Vis absorption spectroscopy by employing the Beer-Lambert law. The π-π stacking interaction of aromatic molecules can lead to additional ring-current effects, which induce upfield chemical shifts of aromatic protons in the aggregate form. Hence, the shift in chemical shifts was monitored as a function of concentration. The obtained data were further analyzed by the isodesmic association model (Equal K model).^(S9) The least-square curve fittings were carried out using Eq. 2,^(S10)

$\begin{matrix} {\delta = {\delta_{m} + {\left( {\delta_{a} - \delta_{m}} \right) \times \left( {1 + \frac{1 - \sqrt{{4K_{agg}C_{t}} + 1}}{2K_{agg}C_{t}}} \right)}}} & {{Eq}.\mspace{14mu} 2} \end{matrix}$ where δ, δ_(m), and δ_(a) stand for experimental chemical shift, calculated chemical shift for the monomer, and calculated chemical shift for the aggregate, respectively. K_(agg) stands for aggregation constant and C_(t) stands for total concentration of compound 10. Concentration-Dependent Kinetic Studies

All kinetic studies were performed without deoxygenating the solutions, and the accurate concentrations were calculated and confirmed through UV-Vis absorption spectroscopy by employing the Beer-Lambert law. The kinetics of BF₂-azo 10 were studied by either UV-Vis or ¹H NMR spectroscopies under different concentration conditions: (1) A solution (2.25×10⁻⁵ M) of BF₂-azo 10 in CH₂Cl₂ was irradiated with 600 nm light source for 1 min. The cis→trans thermal relaxation was monitored by measuring the change in absorbance at 535 nm as a function of time with 2 s intervals; (2) The NMR samples with concentrations at 0.15, 0.53, 1.51, and 8.26 mM were irradiated with 600 nm light source for 2 min. The cis→trans thermal isomerization was calculated by monitoring the change in the intensity of proton H8 as a function of time; (3) The in-situ switching of the isomerization half-life was performed by consecutively diluting and concentrating a sample solution between 1.73 and 1.20 mM. The data was calculated using the same strategy in (2). The observed thermal isomerization in all of the experiments follows a first-order decay process. The double logarithmic plots of k as a function of concentration follow a linear relationship.

Crystallography

A red needle crystal with dimensions 0.264×0.09×0.061 mm was mounted on a Nylon loop using a small amount of paratone oil.

Data were collected using a Bruker CCD (charge coupled device) based diffractometer equipped with an Oxford Cryostream low-temperature apparatus operating at 173 K. Data were measured using omega and phi scans of 1° per frame for 10 s. The total number of images was based on results from the program COSMO^(S11) where redundancy was expected to be 4.0 and completeness of 100% out to 0.83 Å. Cell parameters were retrieved using APEX II software^(S12) and refined using SAINT on all observed reflections. Data reduction was performed using the SAINT software^(S13) which corrects for Lp. Scaling and absorption corrections were applied using SADABS^(S14) multi-scan technique, supplied by George Sheldrick. The structures are solved by the direct method using the SHELXS-97 program and refined by least squares method on F², SHELXL-2014,^(S15) which are incorporated in OLEX2.^(S16) All non-hydrogen atoms are refined anisotropically. Hydrogens were calculated by geometrical methods and refined as a riding model.

TABLE D Crystal data and structure refinement for azo-BF₂ 10 CCDC 1500206 Empirical formula C₂₁H₁₃BF₂N₄ Formula weight 370.16 Temperature 173.0 K Crystal system monoclinic Space group P2₁/n Unit cell dimensions 14.6614(2) Å 6.76028(10) Å 17.1457(2) Å 90° 92.7294(12)° 90° Volume 1697.46(4) Å³ Z 4 Density (Calcd.) 1.448 mg/mm³ Absorption coefficient 0.854 mm⁻¹ F₀₀₀ 760.0 Crystal size 0.264 × 0.09 × 0.061 mm³ 2θ range for data collection 8.128 to 144.404° Index ranges −14 ≤ h ≤ 18 −8 ≤ k ≤ 8 −20 ≤ l ≤ 21 Reflections collected 12563 Independent reflections 3212 [R_(int) = 0.0638, R_(sigma) = 0.0445] Data/restraints/parameters 3212/0/253 Goodness-of-fit on F² 1.027 Final R indexes [I ≥ 2σ (I)] R₁ = 0.0474, ωR₂ = 0.1177 Final R indexes [all data] R₁ = 0.0779, ωR₂ = 0.1342 Largest diff. peak and hole 0.29 and −0.18 e Å⁻³

REFERENCES

-   (S1) (a) Kuhn, H. J.; Braslaysky, S. E.; Schmidt, R. Pure Appl.     Chem. 2004, 76, 2105-2146; (b) Bandara, H. M. D.; Friss, T. R.;     Enriquez, M. M.; Isley, W; Incarvito, C.; Frank, H. A.; Gascon, J.;     Burdette, S. C. J. Org. Chem. 2010, 75, 4817-4827. -   (S2) Flipo, M.; Willand, N.; Lecat-Guillet, N.; Hounsou, C.;     Desroses, M.; Leroux, F.; Lens, Z.; Villeret, V.; Wohlkonig, A.;     Wintjens, R.; Christophe, T.; Jeon, H. K.; Locht, C.; Brodin, P.;     Baulard, A. R.; Deprez, B. J. Med. Chem. 2012, 55, 6391-6402. -   (S3) Xi, J.; Dong, Q. L.; Liu, G. S.; Wang, S. Z.; Chen, L.;     Yao, Z. J. Synlett 2010, 1674-1678. -   (S4) Dai, Q.; Yu, J. T.; Feng, X. M.; Jiang, Y.; Yang, H. T.;     Cheng, J. Adv. Synth. Catal. 2014, 356, 3341-3346.

(S5) Finkelstein, J.; Linder, S. M. J. Am. Chem. Soc. 1951, 73, 302-304.

-   (S6) Ferguson, G. N.; Valant, C.; Home, J.; Figler, H.; Flynn, B.     L.; Linden, J.; Chalmers, D. K.; Sexton, P. M.; Christopoulos, A.;     Scammells, P. J. J. Med. Chem. 2008, 51, 6165-6172. -   (S7) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am. Chem. Soc. 2012,     134, 15221-5224. -   (S8) Kastler, M.; Pisula, W; Wasserfallen, D.; Pakula, T.;     Müllen, K. J. Am. Chem. Soc. 2005, 127, 4286-4296. -   (S9) Martin, R. B. Chem. Rev. 1996, 96, 3043-3064. -   (S10) Tobe, Y.; Utsumi, N.; Kawabata, K.; Nagano, A.; Adachi, K.;     Araki, S.; Sonoda, M.; Hirose, K.; Naemura, K. J. Am. Chem. Soc.     2002, 124, 5350-5364. -   (S11) COSMO. V1.61 ed.; Bruker Analytical X-ray Systems: Madison,     Wis., 2009, p Software for the CCD Detector Systems for Determining     Data Collection Parameters. -   (S12) APEX2. V2010.11-3 ed.; Bruker Analytical X-ray Systems:     Madison, Wis., 2010, p Software for the CCD Detector System. -   (S13) SAINT. V 7.68A ed.; Bruker Analytical X-ray Systems: Madison,     Wis., 2010, p Software for the Integration of CCD Detector System. -   (S14) Blessing, R. H. Acta Crystallogr. A 1995, 51, 33-38. -   (S15) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112-122. -   (S16) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.     K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339-341.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. 

What is claimed is:
 1. A method of controlling the isomerization rate of a compound, comprising: (a) selecting a compound capable of isomerizing between a thermally stable isomer and a kinetic isomer, and wherein thermal relaxation is concentration-dependent; (b) providing a solution of the compound having a preset concentration of the compound; (c) exposing the solution to electromagnetic radiation having a wavelength effective to cause isomerization of the compound in the solution to the kinetic isomer; and (d) shielding the solution from electromagnetic radiation to allow for thermal relaxation of the kinetic isomer, thereby controlling the isomerization rate of the compound, wherein the preset concentration is determined by: (e) measuring the number of molecules of the compound per unit volume (concentration) that provides a desired half-life of the kinetic isomer, step (e) preceding step (b); wherein the compound is of Formula II:

or a salt thereof, wherein N═N—R² can be oriented cis or trans to the tricycle; R¹ is H, CN, CO₂H, CO₂(C₁₋₆-alkyl), C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl); R² is phenyl; and R³, R⁴, R⁵, R⁶, Wand R⁸ are each, independently, H, C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, NHC(O)(C₁₋₆-alkyl) or a group corresponding to a small molecule pharmaceutical; or R³ and R⁴, R⁴ and R⁵, R⁵ and R⁶ or R⁷ and R⁸ can, when taken together, form a fused aryl, fused heteroaryl, fused C₃₋₆-cycloalkyl, or fused heterocycle, wherein the fused aryl, fused heteroaryl, fused cycloalkyl, or fused heterocycle can be optionally substituted one or more times with C₁₋₆-alkyl, C₆₋₁₉-aryl, OH, O(C₁₋₆-alkyl), OC(O)(C₁₋₆-alkyl), NH₂, NH(C₁₋₆-alkyl), N(C₁₋₆-alkyl)₂, or NHC(O)(C₁₋₆-alkyl), wherein said step (c) takes place in a mammalian cell.
 2. A method of controlling the isomerization rate of the compound:

comprising: (a) providing a solution of the compound having a preset concentration of the compound; (b) exposing the solution to electromagnetic radiation having a wavelength effective to cause isomerization of the compound in the solution to the kinetic isomer; and (c) shielding the solution from electromagnetic radiation to allow for thermal relaxation of the kinetic isomer, thereby controlling the isomerization rate of the compound, wherein selecting the preset concentration of the compound comprises: (i) extrapolating from a graph of concentration versus half-life or isomerization rate; (ii) utilizing a look-up table of concentration versus half-life or isomerization rate for the compound; or (iii) measuring the number of molecules of the compound per unit volume (concentration) that provides a desired half-life of the kinetic isomer.
 3. The method of claim 2, wherein the wavelength (λ) is between 400 nm and 1000 nm.
 4. The method of claim 2, wherein the preset concentration of the compound is between 1 mM and 5 M.
 5. The method of claim 2, wherein the solution is in a liquid or a solid state.
 6. The method of claim 2, wherein the solution comprises a solvent selected from the group consisting of a polar solvent, a non-polar solvent, a gel and a solid matrix. 